While I absolutely hate it when one team owns all the bunkers, I can also see from the eyes of an player who is new to "OSF ways", and does not realize that taking the last bunker is detrimental to gameplay. Punishing players who take the last bunker was always something I threatened to do, but I never did. Let's please warn them first, before punishing. Even then, I still wouldn't punish until the player has done it multiple times, as even I have accidentaly taken the last bunker.
And yes, a server spam message is a good idea. Although people don't read the bottom of their screen, I'll add the new message, just so we can say it is official.