Without a complete reinstall / fresh start, there is no way to reset the consumed licenses. These do however adjust dynamically based on the usage of the network. ISE licensing is based around the number of active sessions using a particular feature sets. In the case of base licenses, every endpoint that performs authentication and we see a RADIUS session for will consume a licenses.
In some environments, RADIUS accounting can be missing or not set up correctly leading to ISE not receiving RADIUS accounting stop messages. This can lead to higher than expected licensing usage since active RADIUS sessions are not cleaned up until they time out after five days. A RADIUS stop message ends an active session on receipt rather than pending for the timeout period.
It's expected that if you are running ISE you will be licensed to handle the active sessions the deployment sees. If left too long in an out of compliance state like this, ISE will essentially lock you out of the admin portal and present only the licensing page for remediation.
