Do you mean the ability to make custom teams in the lobby, or once they join they can't change at all?
If it is the first one, do this:
To set it up, you have to go the forces window, and pick use custom forces. Then, make as many forces as you want, (ex. DotA has two: sentinel and scourge) then, just drag the players to where they should be, and then check the box marked fixed player settings.
If it is the second one, I don't know if it is possible
Go to the Scenario - Forces section. Check in Fixed Player Settings. Then under the force that you want all players to be in, un-check Allied and Allied Victory.
Finally finding about some of the bots that are flooding the users online - bytespider apparently is a huge offender here - ignores robots.txt and comes in from a ton of different IPs
Ghan has said he has fixed this. Monovertex please confirm this fix. This was only a problem with people that had signatures in the upper levels like not the special members but the respected members.