1) Locate baseball bat On a more serious note, I'd contact them and ask for them to stop. Barring that call a lawyer and have a fancy letter sent to someone's boss.
Seems pointless really. If you detect someone hacking your servers and your company does not have a network security department where you can report these things, then dust off your resume and look for a new job. Basically, one of two things is happening. Either someone is breaking the law, or someone has been hired by senior management to test your abilities. If the former, then tell your security people and let them figure out whether to get the lawyers involved. If the latter, then tell your security people and get some brownie points for a) noticing, b) acting promptly, and c) notifying the proper people to deal with security threats to your business. --Michael Dillon