This one's popular on MagiStream and Wajas.
All you have to do is give a reason to ban the person above you. The reason doesn't have to be sensible, reasonable, or logical; in fact, the game encourages weird, radical, insane reasons. :3
Example:
"Banned because I failed my Geometry Midterm with flying colours."
Aaaand~ go! :D
(I hope this isn't going against any rules. D: I can't think of any rules I'd be offending, but still.)