In versions prior to 12.1, there were the capabilities to inactivate (lock) a user record, seemingly to prevent them from accessing the system, but without going the extra step of deleting the user.
The functionality however, did not work. (see PM # 9313). This defect was addressed in 12.1.... by removing the functionality entirely. (See KB TIA07566)
This setting was unintentionally placed on the user configuration page.
There is no designed feature to deactivate an account in FootPrints 12.
Since the functionality was already introduced, it should not have been removed, certainly not without mentioning it in the release notes!
Would like to see this functionality re-implemented and fixed.
And as an aside, until such time as this is done, the 'Status' column on the user page should have also been removed: