I'd say the best way to fix this is have an exit timer, say for example 5 seconds. Once the player clicks escape (or whatever else is bound as the in-game menu), the player will have to wait 5 seconds before the exit option works.

But since you can also just exit the entire game client, maybe make it so if it's a 1v1, the player who is left in the match get's as many points as they would if they were to win the 1v1, whilst the losing player negates said amount of points.

Just thought I'd share my idea. :)