I'm the commish of the league in question and I'd like to thank mal for bringing this issue up. It has not been going on for an extended period of time, but might have been spotted over the last 4-5 series. Those other teams may or may not have noticed, but you certainly did, mal, so thanks.
The etiquette called for is pretty open: post in the league chat, trade chat the owner and/or the commish, sitemail the owner and/or the commish. Really, the important thing is not where you bring this to attention but rather how.
Don't presume guilt or resort to namecalling. Give the league the benefit of the doubt until it is clear that others are failing to act.