Protection from ip changers
 

How do you go about protecting your comunity when members can change their ip's? We have a group of members which we belive have signed up under different users. We think they are using something like from this site,
otfans.net/showthread.php?113044-SmygIP-Windows-Linux-ip-changer!-New-Windows-and-Linux-version-v0.06a-with-8.40!

Thanks for any advice.