Use the delete option on the context menu in the Navigator view. Select the symbol, RMB - delete - if there is more than one symbol you will get a list to allow you to delete the required symbol(s).
Unfortunately, as I said, there are more than 200 resistors parts in my library, so when I select "delete" in the navigator window I get the error message: Cannot delete 'R': this symbol is referenced by one or more parts
I made a copy of R with another name. On this copy I followed your instructions and it was possible to delete the .3 simbol.
Now what to do? Any idea?
In that case delete the symbol file R.3 from the SymbolLibs/'partition name'/sym folder and it should remove the reference to the .3 file. As a safeguard backup the library before doing so using zip or equivalent.