Computer system trespassing is a growing concern, but it has received little criminological attention. The present study discusses the results of an experiment which looked at system trespasser behavior after exposure to one of three warning messages (or no message) in the context of deterrence theory. One message consisted of an attempt at moral persuasion; the second a generic legal warning, and the third an ambiguous threat. Keystroke data was analyzed to assess how the type of message affected the employment of restrictive deterrent techniques designed to limit trespasser activity on a compromised system. It was found that moral persuasion generally reduces both the incidence and frequency of command entry by trespassers on an illegally accessed system, while legal and ambiguous warnings produce no significant differences from the control condition. This suggests that in order to reduce trespasser activity, system administrators should use moral persuasion instead of legal sanction threats.