These lockout keys, sometimes referred to as "killer keys", are designed so that once inserted into the lock they can never be removed again. As a result, anyone with a previously working key will not be able to use it to open the lock, the lock will only be able to be unlocked from the inside once this key has been inserted. Works great for locking out evicted tenants until the locks can be changed or any other time you need to quickly lock out someone who has a key.
Some skilled locksmiths can sometimes remove these keys without damaging the lock, but that is very rare. Generally these keys will be stuck in the lock permanently and the lock will need to be destroyed and replaced. These keys will not disable electronic locks such as ones you can unlock using your phone or by entering a code.