Kerry,
That is weird. Normally, when I am having problems with different data sources, I convert to a FGDB item and the problems normally go away.
Normally, if I make sure to delete ANY variable that has been assigned to the dataset or to a map document that contains that dataset (or has EVER contained it, even if has been deleted)(for this reason, I no longer "modify" existing mxd's. always start with fresh ArcMap document, and copy/paste datasets if I want a "copy" as this doesn't carry the links/locks to "old", once added datasets) I can delete them fine. Sometimes the overwriteoutput setting works, but others, you must delete it first.
It didn't look like it in your example, but perhaps you simplified it for posting, but, if you have any of your FC's or tables loaded from a feature dataset within the FGDB, ALL features within that feature dataset will be locked, not just the items currently loaded. You will have to delete any/all variable references to any of the datasets within that feature dataset before you will be able to delete.
R_