I have made multiple team deathmatch maps, and I understand your frustration. When I can finally get enough players to fill it, I see the spawn camping problems and then have to add more spawns. Unfortunately the game uses whatever unknown method to choose the spawn that is used, so all you can do is give it a lot of options and hope it chooses a good one.
The real sad thing is the people who do it. You would think that if someone accidentally stumbled upon a spawn point and realized what it was, they would realize that it is unsporting and leave. But decency is dead in the void of anonymity.