How to prevent banning?

Essentially, I am a dual-spec Paladin. When I am running dungeons or farming them nightly, i typically change my ICC rep farm ring to defense so that i can queue faster. I only do this because I do not have a second tank ring. The only ring I have is Harbinger. Well, this evening a tank ring drops and I need it because I would like to keep it instead of switching my rings for the spec I am using in the moment. Well, needless to say, someone in the dungeon complains that I ninja'd the ring because I have the ICC rep ring and the Harbinger ring. This is not the case, I explained to them that I only have the ICC rep ring on tank because of dungeons, but they refuse to listen and plan on screenshotting and submitting a complaint. What can/should i do to be proactive in these situations?

Edit: I was also the tank of the dungeon. He was a DPS and needed the ring also.

Don't switch rings, you'll eventually get the other tank ring, it's not worth throwing away 200 gold each time.
You can "prevent banning" by rolling the current spec you're doing in the dungeon. If you want to roll tank only items you queue as tank only, if you want to roll dps only you queue as dps only, if you don't care what you roll for (once you don't need dungeon items) queue as both.
Doesn't seem complicated to me...