Wow, threatening a player on here is very brave.

It sounds like he hasn't broken your terms and conditions but you don't like the way he/she played. It is ridiculous to claim it is "illegal" when it was your responsibility too make it clear what is allowed with the bonus. Maybe posting on here is not the right thing for the player to do, but it should not be an issue to resolve problem in public if this is what they want.

If this player really found a bug the honest thing to do would be to pay the withdrawal and fix the issue.