The costumers themselves are not your phycial property, but the paperwork with their personal information (telephone numbers, addresses) is. If you have proof that that was somehow taken, or copied, that just might. My personal feeling is, if someone is going to be that underhanded, it always comes back to bite them in the rear. A teacher before me, did this to my old boss, stole addresses and wrote letters to students saying that her business was closing and that it was requested they follow her. She lost A LOT of students, including most of her senior company. Three years later she had to move locations mid-year b/c she could no longer afford her lease, this year she had no choice but to sell the business. What comes around goes around.