Checkmate

If he won't resign, you have to checkmate him. If you don't have enough material to achieve this, or don't make it in 50 moves (as can happen with a difficult mate like with bishop and knight), or run out of time in the process, or stalemate him, it will be a draw. At least you can't lose anymore.

What happens when you've captured all of an opponents pieces and its left with his king.