Since the Devs don't want to restrict people from going to any map they paid for even if they are level 1 (and I understand this reasoning), the Devs don't want to put min level restrictions on maps. They don't want to do it game wide.
So, why not add an option when creating a game that you can set a minimum level for that game? This way if you don't care if there are lower level chars in there because you could give a crap or maybe you have a buddy new to the game tagging along with you, then you don't set this option.
If you are in the upper maps and want to set a min lvl of 25, great.
If you want an all lvl 30 experience to test how fast everyone goes through all at 30, great.
Maybe you want to help the poor leechers and you call your game Leechers Welcome and don't set the min lvl option.
If this was set for each game, then you would know going in that you didn't have to kick every level 4 that pops in. You've already set the min requirements.
The player attempting to join the game should be told why the join failed if there was the level requirement toggled on and what level he/she needed to be. In addition, I guess there should be a min lvl=xx on the game join screen.
Anyway, just a thought.