The problem is that so far no (FOSS) game has really implemented this, but there are good examples from commercial games where game accounts are linked to CD-keys, making it dangerous to cheat. Of course this doesn't translate well into the FOSS game scene, since free accounts don't offer much of a deterrence, and people are unlikely to pay much (or at all) in this environment.
One of the possible solutions (but untried yet) would be to make the registration invite only, with a limited amount of "keys" to give out. This might work, but has a series of disadvantages and gives a high entry barrier for completely new players.
My new(?) idea would be to make a non-profit organization/website that offers an easy to integrate plug-in for games, and a low payment/subscription of lets say 10€/yr or so (e.g. enough to make people not want to register multiple accounts for cheating) to get a unique gamer id. This id would not only make it possible to ban players, but could also be used to have a personalized user name in all games participating etc... so a lot of additional benefits. On the website you could also host player forums and all sort of other social network stuff that's happening around the games (e.g. game play wikis, dev. wikies etc).
Further more the income could be used to rent game servers, which could also be subscription access only (but the service should be available to trusted non organization servers also), and one could have users vote on the website which games to make servers for, and maybe also to what projects donations could be made.
Once the organization is big enough it would also have a program similar to Google's summer of code, to become a patron of certain projects development.
Since it's a completely additional system, questions of reliability wouldn't arise since games would continue to work as before if the system goes down.
And if the plug-in is licensed liberally it could also be integrated with freeware titles or indi games, but to promote FOSS games some rules could be made that the income is only used for FOSS projects.
One could also make a deal with a indi game that purchasing their game will give you a free account in the system, to further increase the number of users.
What do you think? In fact I might start such a service with a friend who is currently looking to start a bigger website anyways. Have to talk to him first though

last but not least if someone would also run the rent-able server infrastructure as the preferred partner of this organization, it might give you a good additional income with out compromising the non-profit ideals of the organization at all.