Does it grind your gears when you enter a city and a quest auto-adds itself to your quest list?
With this very simple addon you can check the ID of the quest and then add it to the block list.
Unfortunately it cannot stop the quest before it arrives, but it will immediately abandon said quest after it arrives from now on.
The addon triggers when the quest log updates so you'll have to abandon the quest manually for the first time.
/questblocker - list commands
/getquests - List all quests in your log with IDs
/blockquest <ID> - Add quest with this ID to the 'block' list
/unblockquest <ID> - Stop 'blocking' a quest
/blockedquest - Show currently 'blocked' quests