Once you've initiated a mission I don't think there's anyway to go back and uninitiate it. I'm assuming you took on one of the optional missions and you don't want to do it anymore? There might be a way to go around and skip to the next mission after initiating one before it, but if there is, I don't know about it. Might as well just do the mission.

If you're talking about quests, well, you can just not do the ones you initiated and it's no problem. I have dozens and dozens of quests I got by talking to NPCs and no intentions of following through with the quests anytime soon.