Yes, it is highly unlikely theymos would ban signature campaigns but it is not impossible to do. As long as he decides taking a particular course of action is in the best interest of the forum, he will probably do it but thankfully he has not indicated anything negative towards signature campaigns as far as I am aware.
Theymos has once or twice expressed that he's considered removing signature campaigns from the forum, but decided against it due to certain reasons. Here is one of such situations;
Signatures are going to be added back in at some point. Maybe the height limit will be smaller than here, though. I thought about removing signatures, since that'd really clean things up, but I decided that signatures (and especially sig campaigns) are too important to the culture here to remove.
If signatures are ever removed, it would clear up 100% of the spam which goes on here, but would also reduce a number of active users contributing positively, or reduce how much time they spend on the forum.