I'm a little unclear on the question. Do you mean they're stealing the actual unsold tickets? That's a standard security problem that a lock and key can solve. If they're counterfeiting your tickets you need to use tickets that are harder to counterfeit. There are a number of services that will print custom event tickets with foil, holograms, watermarks, and other things that are hard to counterfeit. Just Google "Security ticket printing" and you'll find tons of services. If they're scalping them (Reselling at a profit) then it's harder to stop. A first step would be to not sell tickets in blocks of more than 8 or so, and check the names and credit card numbers before making a sale to keep the same people from buying enough to scalp. A more extreme option is to print the customer's name on each ticket and require matching ID. This will probably require more labor on your part, and may be difficult, especially if your events have kids with tickets. However I know a few small venues who do it this way. One place just uses common raffle tickets, but they take your name and the ticket number when they sell you a ticket and they check it at the door. If you don't match, you don't get in.