Change the surrender option
I say take away the ability to start a surrender and only make a surrender available under certain conditions. If your team has less than 2/3 of the enemies kills, you are missing 1 player, etc.
This will do a few things. It will prevent people spamming surrender just because the enemy team got baron (I've come back from games where the enemy got baron, all inhibs, and all towers), and it will also encourage people to play a game out if it's close,
Honestly, I enjoy full matches more than I enjoy surrendered ones.