I think it'd depend on how strongly they were expressing their opinion. If they'd just mentioned it a handful of times and I happened to see that, I'd just sort of ignore it/focus on things we agree on instead, or maybe debate the issue. If they were being aggressive, posting about this opinion everywhere, I'd unfan.

It'd also depend on what sort of opinion it is. If they just liked a fandom/character/pairing I dislike, I wouldn't mind. I think the vast majority of those I'm a fan of disagree with me on some fandom or other. On the other hand, if they were expressing a more serious opinion I disagreed strongly with, I might unfan.

I guess it depends on the opinion but I have yet to actually do that.

I've never done it on Fan Pop. But I.m a "hot head" and I've done it everywhere else.

I haven't actually done this here, but I've come across a few comments which have made me not want to fan that person/which would make me want to un-fan them if I'd become their fan before seeing.