IP blocking was a waste of time anyway, as problem users either simply switched IP addresses or their ISP did it for them. A much more efficient method is to make your forum more secure yourself.
Just make it so only members can post messages (or you can go further and make it so only members can read the forum in the first place).
You can do that by creating a group. You can add members to the group yourself, if you know their correct username, or you can allow members to join the group. It's up to you. Then you give only that group the permissions you would like it to have. Other visitors will have less access. This is the same method used in every self-installed, self-maintained PHP based forum. It is a bit more work for you as a forum owner, but it pretty much ends the problem user problem.
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