I beleive you can just detach the reference. The only reason why you won't be able to do this is if they are coming in with a referenced file. If they are coming in this way, open the reference file and detach it there.
If it does give you a hard time detaching, you may have to create a dummy image in it's place so that the file is found and then detach that. But that would be an extreme case of it not behaving.
Pieter