As you see on the screenshot I have many meshes in the DataBlock view of the Outliner, but in fact, I use only 3.
How to delete rest of them ?
Here is video describing the problem: https://www.youtube.com/watch?v=RM9Rr1gnL5g
Change the outliner view mode to "Orphan Data" instead of "Datablocks".
To the right of the search field in the header is a "Purge All" button;
and may simplify your problem.
This button essentially performs the save and reload process in one click, which in turn invokes Blenders garbage collection routines.