Here's the deal...if you decide not to use an entity or segment anymore and remove it from your level FPSC now removes it from the library for that level when you save...and freezes.
You have three chioices...
1. You can open the Windows Task Manager and shut down anything FPSC related...generally that would be FPSC.exe and the Map Editor.
2. You can doenload Candles' nice utility (you should be able to find it if you search using my name (FredP) in the searchbox.
When you hit alt/k it shuts down FPSC...you sitll hae to open the Task Manager and close the other stuff but the utility ends FPSC instantly and is designed specifically for the purpose of the save freeze...it is quite handy.
3. You can test the level.This will also remove the segments/
entities from the library and when you exit the test level you can save without freezing provided you did not remove any more media after you exited the test level.
The downside to that,of course,is if you are testing a larger level it might take quite a bit of time to load it.