You can only quit after the next round begins. Just wait until the "Continue" button appears, click the "Continue" button to go into next match then quit. This bug has been with the game since launch.
Remember that about 99% of the game's awful performance issues is caused by the uncapped FPS which they basically refuse to fix. This was supposed to be taken care of THREE PATCHES AGO and it's still not fixed. I just applied today's patch and when I launched the game, it immediately crashed back to desktop. There's no way anyone can be testing this game on a PC and doesn't know about this issue.
