Plummerman, whilst I don't like cats much, there are far kinder ways to get rid of them than to use something likely to cause them a great deal of pain - that's just plain cruel
You could always purchase a cheap car cover if all else fails. Citrus or pepper usually works with cats though,pet shops sell the cat pepper stuff,you just sprinkle it around the boundaries of your property and it keeps them away without harming them or anyone/thing else.