Yes. Just delete that endpoint's account at your RBS Server and the licenses will be recovered for use with another customer.

RBackup's licenses are managed by your Server.

Here's an example - if you buy a 100-License version of RBackup you own those 100 licenses forever. You may distribute thousands of copies of the Client software if you like, and you will not use licenses until someone signs up for your service and you add an account for them at your Server.