I'd remove the need for a log-in to vote on games. Regardless of 'abuse', which should be relatively easy to monitor, if someone feels particularly passionate about up-voting, perhaps they should be able to express it.

Considering a large percentage of individuals who play such games are likely underage, who would want to log-in and, essentially, confess to their crime?

In my experience, sites like this which are 'cool content' driven depend heavily on equally 'cool' design. Perhaps consider design overhaul, find a friend good with web design?