Strategies on banning people

If you have a community site, are there any good resources out there for
how to ban someone if they've broken rules?

You could ban an email address, but they're a dime a dozen.  You could
ban certain types of email (hotmail.com or yahoo.com) but a lot of people
do use them for legit purposes.   You could set a cookie, but if they're
smart, they'll delete it.

Also, are there any resources out there for how to prevent pornography
from being posted.  Say I have a auction site.  How do I prevent some
prankster from uploading an image from a porn web site?

Thanks,
Lara

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