Seriously, there's nothing other than the IP address to uniquely identify a player. And even that poses problems in false positives, seeing how some ISP's rotate the IP allocation to their customers, or how more than one player may try to connect from behind the same IP with NAT. And "bad people" can get proxies.
You can also try to tie accounts to email addresses but it's not like there's a problem getting 100 of those at the drop of a hat.
So if the issue is identifying people perfectly, it's not possible, short of asking them to fax you a copy of their driver's license. The only thing you can do is associate incentives with keeping the same account for longer periods of time, so they'd be reluctant to ditch them. Goes the other way too: make it so that servers can be told to only accept accounts that have been registered for at least a few days, for example, so there'd be no "register account and 2 minutes later I'm server hopping and deconning".