Renlist, I understand your frustration. As noted above and in a previous post, simply defending it is your course of action in my opinion. I really don't see a problem with this and I would be saying to myself, "Wow, I can't believe no one defended this rear flag. I guess they don't want this map to last".
The point I want to make is this - if you're tired of this happening, defend it. And if you keep on defending it, and defending it successfully, maybe these players will get frustrated and quit trying to do this. And If they see you're on during these maps (I always check the player list to know who my/opposing team infy, tankers, pilots are before the map starts), that may be enough for them to decide on a different tactic from the get go. Maybe it will, maybe it won't but you will have done what you can to change their behavior. Perhaps it would also inspire others to help out as well.
I'd be happy to defend the main with you if we are on the same team. What say you?