Originally Posted by erinp:
wind chimes
Flapping tarpaulin
Leylandii
If they're a football, royal wedding fan etc, be sure to mow or strim the lawn when whatever they like is on, if they've got terrestrial TV the mower or strimmer is bound to cause incessant electro/mechanical interference.
Churn up soil a bit and pour small mounds of earth directly over and liberally all over the garden on a regular basis that will give the impression of mole hills. Watch in amusement as they go out and buy poisons, traps and those mole deterring windmill things (the more expensive the better) for no reason.