The problem isn't roleplaying in itself, the problem is that people have no idea on how to do it properly and they are just doing it anywhere and everywhere (even articles).
If people actually made threads for roleplaying then it would be beneficial for both people who are involved and people who aren't since the roleplays will be compiled into a thread or two. More people will be able to see it, the roleplaying will be more fluid and easily tracked since it's in the same place. It will be more fun and more manageable for everyone. It would also last longer since people know where to find it. It wouldn't clog up the recent activity feed either.
Posting roleplay everywhere and then telling people who disagree that they can't stop it just looks like roleplay is being done on purpose to cause disarray on the wiki.
And yes, it actually can be easily stopped, converting message walls to talk pages is one solution and it often significantly lessens the problem, some wikis have definitely done that. Some wikis even resort to making a roleplay wiki and directing roleplayers to that and nowhere else. Then, of course, there's blocking people. :p