Really-really. Just because it makes no particular sense banning them (I mean novice users). They will just register one more time and start posting crap all over again, and we are essentially back to square one. But if you don't ban them, there is still a slight chance that they will try to join a decent signature campaign and start posting sense...
Or will have to start posting sense if they decided to join one
First of all, the bans usually aren't permanent, especially the first time. The ban is a warning for the user to post better. Not banning the users and warning them that their post quality is bad is completely counterproductive. Those users who are shit posting and aren't banned are not likely to increase their post quality of joining a sig campaign because the only ones they can join are ones who don't care about post quality (like bitmixer). Do some cost-benefit analysis. The cost of not banning users greatly outweighs any potential benefits. You can't just hope that people are going to change their behavior, that isn't how the world works.