Stop the person from spreading X idea. Judging from your avatar you should know all about employing censorship as a form of activism. Fire alarm ringing any bells? That was celebrated like a big victory. It can be a valid goal for an RP. Now make the facility guarded of some sort, and you have a goal and a challenge. 

Likewise, not all activism is legal. Greenpeace, anyone?

If someone is going away from the plot to go on their own adventure, people will start ignoring them. The GM won't provide material. The player in question will be playing with their self. The problem isn't the character's ideals anymore, but about being non-interactive and no longer being an asset to the RP... Which really is something that will nearly always solve itself (either by the player adapting or dropping based on this feedback.) Still, that could be for any reason, not activism itself. It's not the subject of activism or the player being an activist, it's bad roleplaying.

Also the reason I call you a narrow-minded jerk isn't for having a different opinion. It's mostly for generalising. And the LOLFREE SUX-topic.