From the FAQ: http://www.wargear.net/help/display/FAQ

"Are alliances allowed?
Alliances in games are allowed and usually make sense from a strategical point of view. However, pre-conceived alliances are NOT allowed or tolerated. Player's that are found colluding in multiple games will be at risk for being banned."

In this case I'd suggest sending them a strong message/warning against doing this again but don't think it consititutes banning the players after one offense.

Looks like it could have been a mistake on their part -- they posted after the game concluded that they'd meant to start a private game, not a public one and just wanted to end it quickly. Seems a little weak to me, given that they didn't bother to let me know that during the game and there IS a vote to terminate game option, but I'll give them benefit of the doubt this time...

...on your own map no less!