Hello,
You can delete registered users and their devices from inside the MAC Registration Administration web interface.
There is an article on a similar topic below, on increasing the device count, but the article will tell you how to get to the web interface I am referring to...
https://gtacknowledge.extremenetworks.com/articles/How_To/NAC-How-to-increase-the-maximum-number-of-...
To do this automatically, please look at the setting in the Captive Portal Configuration: Delete Expired Users". This should trigger (if selected) when the registration expires.
Is this what you are looking for?
Regards,
Scott Keene
Extreme GTAC