I've thought about having something like a "ban amnesty" before as well and mass emailing people in the banned usergroup before moving them to "registered user". We've had over 100,000 accounts signed up over the years, but what I've noticed is most people tend not to come back once they've received a ban, even if its a temporary one. That's why I wasn't really feeling the new infraction system which makes it easier for people to get banned and for a longer period by adding more points.
We have a few options on how to treat troublesome members without banning them.
There is no way (without adding some new coding) to ban a user from battling other than to remove all their credits. To remove credits from a user, just go into your ModCP and where it says "Give Credits," input a negative number. If you type -35, it will take away 35 credits from the user. We also currently have no way of knowing exactly how many credits each user has at the moment. You can sort of guess by seeing their amount of battles voted on in their postbit (if they have any forum posts) and comparing that to the number of battles they've dropped. Either way, if you minus a very large number of credits, say -4000 from any user, it should ban them from being able to post battles and even if they earn credits by still voting, it will take a while to recuperate (we can also simultaneously lower their vp to 0 to make their vote not count). Then you can just manually re-add the credits you took away when their punishment is over.
To ban a user from using the forum, we can either create a new usergroup with no forum access but which can still battle and stick them in there, or we can somehow make all their posts moderated so that it requires approval before anyone sees them.
We have one more option and that is the "Miserable Users" usergroup. You place someone in there and they'll be able to do practically nothing on the site because everything will be so slow and take forever to load. They'll think the site is glitched or something. The only person I've ever placed there was Rohan when he kept making new accounts during his ban. He couldn't make any posts or drop any battles or anything. The taking away credits option, you can do on your own if you feel someone deserves it. The other options of changing usergroups would require myself or another admin to do it. Just let me know when you want to punish someone that way rather than ban them.
@
Paperclip
EDIT: I just realized by removing credits from my other account that it only allows you to minus a 3 digit number amount of credits at a time so the most you can take away is -999. You can do that more than once though