Thanks, I forgot to take into account networks. And since I actually have a lot of users coming from school networks, I can't use my current approach.

usernames & passwords aren't a viable option, mostly because it would add an unnecessary layer of complexity and generally be overkill for the problem.

I never really liked cookies, they're too easy to circumvent (and should be), which comes from having tried using them before when writing a crap Hotornot clone.

I guess these are the only solutions, though, so I'll see which one will be the best.