You can use this Avenue script to remove icons from the Icon Manager.
'-- Script: "icondel.ave"
'-- This script allows the removal of icons from the Icon Manager.
'-- Get the list of icons from the Icon Manager
IconsList = IconMgr.GetIcons
'-- Select an icon to delete
ToBeDeleted = MsgBox.List(IconsList,"","")
'-- Remove the icon from the list
'-- End of script "icondel.ave"