I did the "Two Sides of a Coin" quest a couple of days ago and this evening, Myvryna Arano showed up inside the Fighters Guild in Cheydinhal and gave me a "Mysterious Letter." My Altmer Knight is far too honorable to ever consider joining something like the Thieves Guild. I'm wondering, because its been a while since I did the Thieves Guild, if I can go to the waterfront and tell Armand Christophe that I don't want to join the Thieves Guild? Its kind of annoying to have that letter perma-bound into my inventory, and I don't like having a quest in my journal if I don't plan on finishing it.

I play on 360 so I can't use any commands to get rid of either the letter or the Quest.

Thanks in advance for any replies.

No, you'll have to live with it. There are plenty of little things in any RPG as deep as Oblivion that you just have to ignore if you're roleplaying.

Join and get kicked out,the mysterious letter will be gone but the quest will remain...just don't do it and it will move to the bottom of the list.