Go to hunting supply store. Purchase hunting scent (Skunk Urine). If possible, buy the one that is oil based. Get creative, but make sure that you wear rubber gloves. If this stuff gets on you, you will smell like a orangutan cage that hasn't been cleaned out for a month.
Recommended locations would mail slot into house, open windows, base of car windshield. If they have a dog or a cat, you could put it on them and let them do all the work for you.
Trust me. This stuff is brutal. If you use the oil based stuff it will take them weeks to get rid of it, even if they can find it. They actually will seriously consider moving out.