Good idea, I'm sure it could be useful it a handful of situations.
If a co-mayor of a large town chose to, they could set the taxes to a large negative number which would destroy the town.
~ @SuperDyl
Valid point, but this doesn't benefit the co-mayor at all, and mayors can probably pick their co's more carefully than that.
As for further action beyond a ban, the co-mayor can be fined and the town refunded. There are very few ways to use this to break the rules, and all of them are detectable and manageable as far as I can see.