Don't the resolutions have a time limit? After a few years it should be over.
If it's permanent, then you can either leave the UP (which prevents trading, so not a good idea in this case), or find other means of income. At least any AI that was planning to do heavy trading is screwed too.
Dooey - You cannot reverse votes. Also, you cannot yourself propose anything - you can only vote on what is proposed.
Morpork - It depends on the resolution. Some are permanent. But you're right - should you leave (and your Empire be big enough) the AI screws itself as much as you. Plus you get to break any current trade routes it may have with you, without affecting an Alliance - good news if you're about to attack your "partner"!
Roundball - Good question. Never tried it. Maybe Ruff or ZaH knows...