There are actually a number of hacks available which combat users using proxies.
This one converts the proxy ip to the users real ip and allows vbulletin's IP address functions (including IP address banning) to function based on what the hack determines as the user's "real ip"
https://vborg.vbsupport.ru/showthread.php?t=120082
The other one works by merely banning all open proxies outright... in my experience taking this action is extremely effective, however, it negatively effects just about anyone without a static email address. (When I implemented it, one of my most loyal members could not access the site for a month because they used AOL... it wasn't this particular hack but a vbulletin 3.0.0 alternative though.)
https://vborg.vbsupport.ru/showthread.php?t=96318