Can I license my code?

I've created some software that I use for my own benefit (lots of code). No UI, no web app, etc, it's just code.

I work with a company that's interested in licensing this code from me, but I don't know how I would protect it once I share the code with them? Since there's no web app / API / etc I can't just revoke access if they stop paying for the license. Has anyone ever licensed code like this before?