I am aware that you already have a method to prevent logon of banned users and to monitor who created an object (though you may not have the ability to go through and search for all objects created by a particular logon).
I propose a felon tax. When a logon or character are observed breaking the rules flag them as a felon. Where the felon flag adds a tax on all objects rezzed that would be charged to the felon's account.
That way as soon as a self-replicating attack is noticed a flag can be set to bankrupt the attacker, also it could be set that unless the tax is paid the object is not rezzed.
Or perhaps limit felon rezzes to temp objects only.
Something along these lines would keep as much functionality in the hands of the law-abiding citizens as possible while properly punishing the destructive elements.