People spend too much time worrying about the cheaters. Its nearly impossible to stop them, anyway.
My opinion is focus on what you can control, like the UX, the UI and the content.
Just an opinion, so don't blast me.
But, something as simple as
Code:
select count(id) from ips where ip = $CURUSER['ip']
If (count > 1) die();
This is basic and pretty simplistic but it should give you an idea of how to do it.