I like Utoo's suggestion and the additions of several others (nbcrusader, u2girl1978, and others) and would like to add that if someone attacks a mod, even through email or PM, he or she should receive a warning for that as well, and those warnings should count towards a banning.
For example I think the rule should be you get three warnings in a 12 month period. If you keep doing the same stuff after the third warning, you're banned for a month or two. If when you are allowed back you are good, alls fine. But if within a certain period of time (say six months) you get another warning, you're out for a year.
I do appreciate the flak you (mods) get for a banning of a popular member and some of that is just going to come with the territory. However, if people get abusive, even on email or PMs, the same rules should apply. Three warned episodes of bad behaviour -- no matter where on the site or to the mods or other members -- within a year, and you get banned.
And as far as people not responding at all to PMs or emails from a mod -- to me that's the same as a big "fuck you." If the person suddenly cleans up his or her act after an email that's one thing, but if the person doesn't respond or change behaviour, then tough twinkies, after three warnings you're out, whether you respond to the warnings or not.