Is it possible to change they key for a permission to an EOS public key equivalent of NULL? Such that you could in essence stop that permission from being used in the future?
If so, what would be the public key to do this?
The permission that you would set your account to would not be a key, but would instead be the account eosio.null@active
.
This is a special permission that can not be used by anybody ever (it has no associated keys and therefore can not be hacked).
Note: This permission is implementation specific, so whilst it exists on the EOS mainnet, it could be that it doesn't exist on a sidechain if they chose to remove it.
A little late to the party but the NULL key is:
EOS1111111111111111111111111111111114T1Anm