A big key to that is to reject free email sign ups, because it makes it just painful enough for them to sign up another shill that it's more work for them to find a new unbanned free email provider than for you to ban them.

If you can shift the work so that they pay a higher price to come back in time and energy than you do to ban them then they get tired before you do.

It does limit participation a little as some legit members would like to sign up with their yahoo or gmail account, but I've found that it hasn't been that much of an impediment.

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