Swap their #2 plug wire for #3.
Long zip ties on their car's drive shaft.
Wait until Winter, then take one of those rear diff lube syringes(trust me, it makes this MUCH easier!) and get some water and a valve core wrench and an air tank or pump, hell a can of fix a flat, anything to pump 2 tires up. Go to opposite corners of their car, and fill those tires with about 2 quarts of water each. Pump each one back up, and go home secure in the knowledge that their morning commute will most assuredly suck!
This one's for warm weather;
Dehydrate yourself a little so that when you pee, you pee a nice, stinky, golden yellow.
Then pee in a cup and pour this on their bikes headlight. The next few rides will suck as well.
A more extreme smell output is gotten by pouring your pee on their engine and headers.