You should log in to the Web interface to delete the key that way. You can't authorize as a Super User to the PGP Universal Server from the PGP Command Line product, it was never intended to perform server maintenance operations. The funtionality that the two were designed for is simple key lookups and key storage.
If you aren't the owner of a key (i.e. you don't have the private key material), you should not be allowed to manipulate that key. The server admin, logged in through the Web GUI, will be able to make that administrative change (remove the key).