@Cratoon, if you are completely certain that he abused your system and can prove it than you don't owe him anything. If he cheated why are you offering him a $100? Just lock his account.
If you can't prove he was cheating, you have to pay him in full or at least talk with him and try to reach a compromise.
Also, can you, please, explain what do you mean by this:
Well if you haven't deposited anything, you haven't lost anything either, innit?
See, the thing is we discussed with him (in our main topic) that we are willing to offer $100 bonus if he explains how he was betting to analyze the situation.
He filled his part of the deal by sending us a support ticket (although no useful information was presented in it), so that's where the number comes from.