You're right - it only remains hidden until the project is closed.
The same thing happens with Jeff's suggestion of sorting by "Used" in the Details view of Media Library, of which I was also unaware. It is set only for the current session.
If you want a more permanent way of hiding/separating used clips, just open explorer view - add a new "Unused" tag and drop your unused media in there...
Cheers - Tony
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