How do we prevent theft of themes ?

I posted this job in the job section:
https://premium.wpmudev.org/jobs/add-second-sidebar-to-a-classified-theme-not-difficult/

a certain gentleman responded that he can do it. He requested I send my classified theme which I paid over a $100 dollars for to him so he can do the job. I haven't heard back from the fellow since.

How are we supposed to get modifications to themes down if we send our themes and then never hear back from the dev pros ?