As I understand it, both the Bitmixer bot and marcotheminer's bot are already able to distinguish text in quote boxes from normal text. With just a few modifications, it should be possible to change it so that it automatically ignores all text that isn't clearly visible. For instance, text that is under a specific size should not counted, as well as spaces over a certain length. I'm not sure if there are any other tricks that could be used other from those two (e.g. there doesn't seem to be a way to change the font color to one that is invisible or to hide text behind a picture).
EDIT: Are embedded images also counted as characters? How about URLs? If they are counted then it's possible that someone could insert 1x1 pixel images or single character hyperlinks to their posts to make it look as though it's longer than it actually is.
The 75 character minimum was likely instituted as a way to prevent people from spamming their signature with a lot of insubstantial posts. It was probably not anticipated that people would try to cheat the system (or maybe it was and the parties didn't do anything to prevent this).
I think it is interesting that bitmixer has almost the exact same rules, yet they have not encountered similar problems. People have complained that they were "unfairly" banned/removed from their campaign so it should be clear that someone is watching in the background even though they are not active in the forum.
The rules for the Coinomat campaign were directly copied from/inspired by Bitmixer's rules which I agree has worked well so far.
However, Bitmixer's campaign has a lower payrate (except for full members), a lower maximum post limit (50 posts), and doesn't count posts made in the alternate cryptocurrencies and games & rounds sections (in addition to the off-topic section) so it's possible that spammers might find the campaign to be less appealing.