My suggestion is to show all the banned users + the reason that warranted that ban (may it be perm or temp)
If there's no plugin availble, maybe you could do a monthly topic showing who was banned and for what reason (link to dispute if required)
This is just for book keeping reasons. I've bought and sold a few things to now banned users, some of which I can't get a reason to why they were banned (and at risk of accounts/payments getting recovered). I'd kinda like my paranoia to be put to rest by knowing what they was banned for.
Also maybe it'll be a good way for the community to see what people are getting banned for the most, and show us how we can, as one, make this place better for all of us.
And now for a more controversial suggestion:
Included in the above suggestion, any convicted scammer's original registered Email address is posted publicly.
If there's no worry about irl repercussions, people will scam. People will still scam by creating fake emails sure, but it'll maybe make people think twice before doing it. Especially if you keep logged the original email they used to sign up with (easy to do guys cmon just another db field and a modification to the sign up page)
I know these are very tricky suggestions to agree on so I'm open to criticism + suggestions.
If we do it (the most active osrs site) maybe the other sites will follow suit aswell.
Edit: @jackshow had a good suggestion that I think it worth adding to the OP. He suggests changing the banned group to something else where we can still rep/forums posts. Please check out and read his post here: http://osbot.org/forum/topic/110632-show-banned-users-reasons-for-ban/?p=1243309