When we receive a few complaints about the same players consipiring together in a ranked game, we then go watch a few of those games ourselves. This is a time consuming task but we think it is worth the effort to pick out the unfair/cheating players.
When we have watched a few games, we might conclude everything is ok. In that case the players in question will probably never hear of us. If we have determined foul play (favoritism or plain cheating), we may do one of the following things. In extreme cases, where the cheating is so obvious that the only goal seems to gain as much points as possible, we may ban immediately without warning. We feel players that cheat that bad, are not welcome at our website at all.
Other times, we think a ban is not immeditatly necessary. Then we may issue a warning to players in question not to play each other in the ranked lobbies again. By giving this warning we can make sure no foul play between the players will occor. We simply haven't got the time to keep watching their games to see if they will still be cheating. This warning includes any other names the players may have, so not just the main player names! In case the players really really only want to play with each other, they can visit the non-ranked lobby.
If the warned players keep playing each other in a ranking lobby, we will assume (based on previous observations) there is still foul play. In this case we will ban the players from our website.