First of all, we don't ban "users". Never have and almost certainly never will. It's very important that this be understood. Anyone may always "use" the Open Directory (that is, either pick up the data and publish it according to the license; or browse the data as published at dmoz.org or elsewhere).
People who do those things are users, and they aren't banned. Clear?
Now, CONTRIBUTORS can be (and are) banned. And this would include either editors or suggestors. If someone is wasting the volunteers' time cleaning up after useless (or abusive) actions, then we're better off without them, forever. The ban is not to punish the jerk, and we can't punish the jerk. All we can tell him is that we don't need his help, and he doesn't have privileges to help anymore. The ban is just to protect the volunteers.
Now, site bans are different. And there, almost nothing is forever. The fact is, domain names expire and get picked up by some other person. And it may be the same name but it's not the same site. Again, we care nothing about punishing ordinary spammers. All we want is to be free to work on OUR goals without them BOTHERING us. For the most part a "ban" means "this site has already been reviewed enough. No volunteer never needs to feel an obligation to ever review it again." But that's not really a "ban" just a "designated bother." An editor can still visit the site if he wishes, or even discuss with other editors why it should or shouldn't be listed.
(But don't ever buy a domain name from someone who tried to threaten or bribe an editor...)
There aren't really rules, except (as I think Jim mentioned) "He who games the system, loses." And that because the ODP is a social process and being a jerk is just flat not a socially effective strategy. It's not because there's a rule against being a jerk.