Spyware and Trespass
As Walter Block, Roy Whitehead and I argue in a forthcoming law review article, "The Duty To Defend Advertising Injuries Caused By Junk Faxes: An Analysis Of Privacy, Spam, Detection And Blackmail" (and as I have argued elsewhere), spam and related activities can in principle be a crime—a type of trespass—since it is a means by which the spammer uninvitedly uses another's property.
A classic case is CompuServe v. Cyber Promotions, which held: "where defendants engaged in a course of conduct of transmitting a substantial volume of electronic data in the form of unsolicited e-mail to plaintiff's proprietary computer equipment, where defendants continued such practice after repeated demands to cease and desist, and where defendants deliberately evaded plaintiff's affirmative efforts to protect its computer equipment from such use, plaintiff has a viable claim for trespass to personal property."
Now, as reported by my former partner, noted cyberlawyer Eric Sinrod, a court in Chicaga has ruled that the doctrine of trespass to chattels also "applies to the interference caused to home computers by spyware." Good.