You can only deactivate Time-Based Keys as per the Cisco Documentation. The permanent key must be replaced with another permanent key with fewer features.
The activate and deactivate keywords are available for time-based keys only. If you do not enter any value, activate is the default. The last time-based key that you activate for a given feature is the active one. To deactivate any active time-based key, enter the deactivate keyword. If you enter a key for the first time, and specify deactivate, then the key is installed on the ASA in an inactive state.
The documentation also states:
You can install one permanent key, and multiple time-based keys. If you enter a new permanent key, it overwrites the already installed one.
Thus, if you wish to revert, you must overwrite the current permanent key with another permanent key with fewer features. If you do not have such a key on-hand, you will need to contact the Cisco License Team. A user in another forum reported that they were able to accomplish this successfully.