Hi,

joined a 2vs2 with predetermined team lately of RoR. Team one was already full, I joined team two. Then the game started as 2 vs 1. They did have no interest in voting and I lost.
Shortly after I see they have a new game open. Another player joined, same thing happened. They have this system of cheating.

Of course i have them BL, but, is there a way of sanctioning/warning/deleting their accounts?

If was a quickmatch game all that you can do is ask for them to deselect the template from QM: Multiplayer > quickmatch > templates.
Else, nothing, it doesnt break rules but is ethicaly wrong.

K, thx

If you believe players are actually cheating by deliberately joining separate teams and then abandoning one, you could also report the players (linking as much info and explanation as possible, since cheating is hard to prove)