I know you can fail quests by playing through them. However, there are quests I just don't ever want to complete that I'd like to get off my Journal screen (in particular, the Thieves Guild quest). I had Join the Stormcloaks until I joined the Imperial Legion, at which point i got a Quest Failed message about the Stormies. Is there a way to console a quest line to failure? Or will I have to know the quest stages? Any ideas would be appreciated.

Note to Bethesda and modders: I would really like the option to delete (fail) quests on the Journal page (with a confirmation dialog of course!)

May be against your morals... but can you kill the "quest giver" or other VIPs related to the quest?

I'd be happy to kill any member of the Thieves Guild. That's a good suggestion, thanks!

You should fail a quest after attacking the quest giver even if he/she can't be killed. Did that in Solitude by attacking Jarl.