Dont know which flow you are using.
I am using the DxDesigner/Expedition flow version 7.9.1 so my solution is based on this.
As far as I know there is not library tools based way within Library manager to delete the symbols. You can delete parts and cells and padstacks but the is no big X to delete a symbol.
Therefore the only way that I implement this is by going into the folder within the library (using windows explorer) where the symbols are and delete them either individually or in a group.
The symbols are located in a folder called SymbolLibs, then you will have a folder whose name you have defined (in my case for amplifies component symbols this folder is called 'amplifiers') then within this folder is a folder called sym.
This 'sym' folder is where the symbols are located and you can delete as you require from here.
Attached is a picture showing located within my library.
Symbols.JPG 45.3 KB
If using the netlist or PADS flows just delete the symbols from the folders where they are stored on your computer.