Forcefully remove database from mirror

An IRIS.DAT file was removed, as it was not needed anymore. But the database was mirrored, so it still shows up in the mirror monitor and database list. How can this be fixed? There is no backup of the .DAT file so it cannot be restored.

w ##class(SYS.Mirror).RemoveMirroredDatabase("/mydir/")

throws a protect error.

(everything else works fine and this is not a production system, it only is annoying in the mirror monitor and database list)