Tdawg, et al...what about this: all accounts remain free, but if you're banned, you have to email me, state your username, and then make a donation in order to remove the ban? This gives the incentive to behave yet no costs upfront.
No. Stastistically speaking, people who are banned will just be signing on again without telling you they were banned prior. There's more incentive to do that than to disclose a prior ban and offer funds to lift it.
Being fine with a modest donation prior to entry demonstrates some support as to why you are doing that on the part of the person wanting to enter the venue. Although it cuts down the number of sign ups, it cuts down on the amount of babysitting the board you have to do too; cuts down on the work load.
If they are in already with some kind of acceptable track record, there should be no worries about a change of policy applicable to new entries.
Just make clear in the policy that those who were banned prior need to disclose past screen name even to be considered for re-entry with contribution. Basically, you want it modest enough so it's easy for you to DECLINE. It should be an affirmation that the person supports what you are doing.