Specify it in your proposal....get programmer to agree to it before you award the job.

If you are looking at cheap then you will be using someone overseas, and even if they agree and then sell it to somone else you will have little recourse as legal costs would not be prohibitive....and at most elance might ban them.

If you are truly concerned then choose a programmer from your native country.

We find in general a better quality of programmers at rentacoder.

We had to sue onew programmer and did it successfully, but that was because he was located in the US.