[Davros]

I think that trying to block people from visiting sites that you do not own would be considered as hacking, if it is not illegal it is certainly immoral.

I have had similar problems with rival forums.

The best way to go is to install Multiple Login and Detection Prevention. Once you ban a user, rather than add them as a miserable user, go to VB Options>User Banning Options & Ban their IP & Email Address.

From that point on Multiple Login and Detection Prevention will automatically ban anybody that tries to register another account, by detecting cookies and other variables, Multiple Login and Detection Prevention usually spots people who are trying to register using Proxies. Of course this is not fool proof but it certainly helps from an Admins point of view providing you configure it correctly.