What about fitting a solonoide to "Bang" the Heli Deck, assuming it is metal, this would cause it to vibrate every so often, they may not like the noise or feaing through there feet.
Or you could use Feonic type actuators to vibrate the deck at varing ferquencys to scare them away, maby every 10 mins, Use solar to charge a battry pack if no power is avalble etc
Nigel