My quote notifications turned on mysteriously yesterday, as well, but I was fortunately able to find the setting to turn it off.
1) Go to settings.
2) In the left-hand Control Panel, there is a "User Tagging" section at the bottom. Choose "Settings."
3) The 6th option is "Hide Quote Alerts." That controls popup alerts about being quoted (I think).
4) The 10th option is "Disable Quote Emails." Just make sure that's checked.