My disliking of Spike is made up of two different elements, The character and (a large group of) his fans.
Character
I liked Spike when he was a baddie and as a neutered baddie. After that his character became incredibly boring. So I dont like the character.
The reason I dont like him from season 5 on, is he loses anything that makes him a character, he has no backbone and just becomes Buffys lap dog.
In season 5 he is literally a stalker, end of.
Season 6 he is a vital element in the destruction of a great character and he is directly involved and responsible for a number of the most disgusting parts of the story.
Season 7 is just awful, he gets a soul that is basically nothing more than a get out jail free card to excuse an attempted rape. Other than minor characters changes he remains largely the same. He has no problem taking credit for the good he did as a vampire, while at the same time he forgives himself for the bad.
Spikes story adds absolutely nothing to the overall story, he has no part to play, his character is too directly tied to being Buffys emotional slave that he has no room to grow and no room to have a story.
Willow has an arc in season 7 thats tied to the overall story, she has to overcome her fear of magic and what she was, and ends up playing a role in changing the course of history for the slayer line.
Anya has to deal with with being a demon, then being made human, which puts her back in a similar place to season 3, when she's so afraid of being human she runs away. We see a complete 180 from that point of view, where she wants to help and dies doing so.
We see more of Dawn and Xander accepting their place in the group, yet no big deal is made(which is kinda their thing) of the fact that they go into battle as humans with no magical assistance.
Spike wears an amulet like a mannequin could do. Absolutely he played the biggest role in that battle, but the difference is it wasn't something he uniquely could have done, and it wasn't something he wouldn't have done two and a half years earlier. He'd have worn that amulet since halfway through season 5, which negates any need for all the screen time Spike gets in season 7.
It wasn't some big heroic gesture that was spurred on by his character development of having a soul, it was something he would have done presoul.
Which is why I can't wrap my head around why he got so much storytime for such little development. That storytime was desperately needed for the overall Slayer story and was wasted for no good reason on Spike.
I say wasted, cuz we have to see him deal with going through being crazy for a hot minuet, then being captured, then the unchipping, his gentler phase, followed by his "I can be the big bad again phase", then the trigger story, then the untriggering story, then his I'm the only one that believes in Buffy phase and on and on, yet none of it plays a bigger role.
His Capture lead to nothing, unchipping him only played a part in separating Giles and Buffy, something that was largely ignored by the final episode, Same for his I can be a big bad again, him putting on the jacket, makes no difference, he's no different from that point on then he was at the start of the season, the trigger story gave nothing but backstory to something not tied to the overall story, the last time we got Spike backstory it was tied to the slayer line, this was pure Spike backstory with no relevance to the story. His sticking up for Buffy when no one else does is kind of pointless, she decides to go off without him to get the scythe, and then goes back to the group and completely changes how she leads basically saying that how she was leading was wrong
All of that in a season where the bad guy was an astronomically big bad guy, and the story was about the very subject that the series is based on (slayers) and both went criminally under developed. Thats maybe a bit outside the character himself, but none of the stuff that took up screentime was interesting, the fact it took away from actual interesting subjects just made it more annoying.
What makes Spike worse is that he can grow, when he went to Angel he was much better. Much better as a character and as a person, but he went straight back to being Buffys lap dog by season 9.
The Fans
This has largely been covered by most other people, and I agree with everything they said.
Spike fans can be incredibly difficult to engage with, not all (but many).
The main reason being there is this need for Spike to always be in the right and always be perfect, that no other characters fans do in the same way Spike fans do. There are a stream of uncanon excuses and explanations that are 100% fanwank and often go against canon, that makes debating someone next to impossible. Even stuff Spike does unsouled fans try to find a way to make an excuse, denial that Spike is the doctor is a milder one, but I've seen people say the attempted rape was Buffys fault - What possible reason would there be to go against those?? At the time of both he was unsoled, the answer to either of those should be "yeah he's responsible for that, he was unsouled" THATS IT, finished. There's no need to pin blame on some grand scheme of Rileys to frame him, or blame Buffy for leading him on - and they're just two examples, there have been many many more.
Another reason dealing with his fans is frustrating is cuz of inconsistencies.
Firstly cuz there's inconsistencies with Spike, some people think he's more capable of love and being human as a vampire, while others dont accept that.
Secondly there's inconsistencies in the points, some fans take "good" Spike as the whole character basically from season 5 on, they point out all the good he's done, but when a bad point is brought up it's immediately "he was unsoled" - how that works I have no idea.
Another fan will only count from season 7 on.
And a third type of fan will say they only count from season 7 on, and in another thread will start pointing out all the good he did in season 5 and 6.
Absolutely other fans defend their favourite character, but the arguments are more consistent.
Another reason his fans can be annoying is that if you dont like Spike you are a hater. I dont like Spike cuz his uninteresting stories took up too much screen time from other more interesting people and stories and I'm a hater. I really dont like Spike, but that doesn't mean my points are invalid, and every time someone does it they just look like such a fool who can't back any of their arguments up.
One of the main reasons I dont like Spike fans is cuz of their effect on the story. Spike fans have always seemed like a loud group of fans rather than a many group of fans. Spike individual properties have never done well (least selling comics etc), yet Spike got so much screen time in season 7 cuz he was popular. I dont like the amount of power a small group of fans seem to have over the story.