Why Can't I Delete Function Blocks? - CX-Programmer

A function block can only be set as no longer used in the Ladder Diagram. Otherwise, we get the following error message:

If the function block is no longer being used, then the next pop up will ask you if you want to delete it (version 6.0):

This means that the name is still present in the Symbol List. 

In version 4 and 5 of CX-Programmer you must first remove the name  from the Symbol List before the function itself can be deleted.