avoid contractor exploiting tkinter software…

I share my "software" that's created in tkinter to a contractor who does my client's service. Does anyone have any tips to avoid the contractor exploiting the software and use it outside my client's service? (aka use it for themself).

I was thinking some kind of one-time-password that I send them everytime they need it. Or maybe some password that let the use the software for x amout of time? Is this possible, or is there a better solution?