Or, click on "no draw", which is right next to "accept draw".
Draw Offer
You can click to decline if you want, but you can also ignore it and do nothing until it's your turn to move, and then move. I think the misconception here could be that the game cannot continue until you've decided to accept or decline the draw- it actually continues whether or not you click either. If your opponent offers draw and then never moves, he'll lose on time.
How do I decline my opponent's draw offer?