just log everything s/he does, make sure you tell his/her ISP first, give them a day or two and if they don't do anything then go to the proper authorities, im not sure about the laws over there but i think they are a little bit softer on the computer related crimes, after all thats where some of the best crackers are, just because its harder to prosecute them.

a common mistake made by even the best...

Hacker - 1.One who is proficient at using or programming a computer. 2. A person who enjoys exploring the details of programmable systems and how to stretch their capabilities. 3. One who programs obsessively. 4. An expert at a particular program.

Cracker - 1. One who makes unathorized use of a computer, especially to tamper with data or programs. 2. An individual who attempts to gain unauthorized access to a computer system.

as you can see, what most people refer to as a hacker is really a cracker and most people dont even know they're wrong