Uhhmmm... No?

Firstly - cleaning a computer of spyware is just a geek duty. It takes no time and charging someone for that is just... Bleh. Even if you're a company that does nothing but computer support and such, this should be a free service to keep clients coming back.

Secondly - you'll get in deep for installing anything remotely harmful or unwanted on this persons computer. Hell, just posting about the idea that you are considering it is incriminating enough to get you sued.

Thirdly - you learned your lesson - cash upfront, but you're not getting back your pride. Swallow it, move on and make better decisions in the future. Retalliation is not going to happen and deffinitely not from the assistance of anyone here.