I attended Tibetan Buddhist Mahayana teachings given near to where I lived a few years ago, and this sort of question cropped up occasionally.

The question sort of went, if I step on an insect accidently, will it incur a karmic consequence?

The answer it seems, is to do with intent (and this is fundamental to understanding karma). If you step on an insect by mistake and kill it, you would not accumulate negative karma because you did not intend to kill the insect (interestingly, the insect could have paid off a karmic debt during the suffering it experienced when being stepped on - you may have inadvertently done it a favour). However, by spraying fly spray or steering them into one of those insect zappers, you intent to harm and cause suffering to the insect and you will accumulate negative karma for that action.

So, to answer KimC's original question, you would have to find an alternative (or put on some insect repellant). pdq's monk seems to be cheating to me, but perhaps I'm wrong.

Free Tibet!