Does it work if you have Debian, deb-multimedia, another third-party repo, and you just want to remove deb-multimedia?
If you have other 3rdparty repos, you'll also need to set 1001 for all of them (except DMO), that should be enough.
And of course strictly speaking it may be unsafe, but I think it's OK in case of debian-multimedia...
-- With best regards, Vitaliy Filippov