I'm the Guild Master of the TG and the Listener for the DB. I'm fine with that. But for the MQ I had to join the Mages College, or if I didn't HAVE to, I did anyway, and now the "First Lessons" quest is in my log forever... I don't wanna do that as well, it'd feel weird being the Arch-Mage. So, question title basically.

Walk out the door.
You can't remove quests from your log once you trigger them.

Edit
Unless you complete or fail them.

I have yet to find a quest that I can fail without dying. Doesn't mean there aren't any, of course. Just that I haven't found one yet.

Anyways, there is no way to leave a guild or remove a quest from your log. Once you're in, you're in for life.

Kill a member? (I honestly have no idea.)

Yeah, killing a member takes off the quest but add a new quest to "rejoin the college" or something to that effect.