RDP User Licensing.... move to new user?

Hi All

I have a Windows 2012 RDP server with 3 RDP user license CAL's. All 3 people have connected and are working, so the RDP shows no licenses left to allocate, which I'd expect. Shortly, 2 of these users will leave the company and I need to free up these two licenses for use by two other people.... how is this done??

thanks!

rob