No. Deactivating or permanently deleting a user or permanently deleting a session doesn't undo usage that has already been counted for your license. Whether your license is based on sessions or on users, sessions that were taken and users who were active keep counting, even if you deactivate or delete those users later.
Should I still deactivate former users?
Yes, if you want to keep your user list tidy:
Deactivated users can't sign in to assessmentQ anymore.
Their results stay available in the reports.
You can reactivate them later if needed.
Only delete a user permanently if they need to be forgotten. A deleted user is anonymised and can't be reactivated.
Related article: Deleting or deactivating a user