There is actually a help article within Protect that covers this process completely:
Essentially when you deploy to a machine the license seat is stuck with that machine name for 45 days. If the machine is not deployed to again for 45 days then the license seat is freed up for use with another machine. There is no way to manually reset the license seat.
Also just a note - you can see what machines currently have a license seat assigned to them by running the 'Seat License Status' report from Tools > Create Report.
Very cool. Thanks for the information Adam.