All good practice, Weaver.
Another closely telated problem of course is harvesting of phone numbers. And sometimes, websites won't take an order, or a hotel booking, unless you give a phone number, "just in case there is a problem". Rubbish. They want your phone number because it is of commercial resale value, especially as they will also be able to sell the name and address associated with it.
My own tactic then is to simply omit the last digit. Much better than giving a made-up number that might actually belong to some other poor soul, yet still ensures the data harvester will be frustrated.