The default right-click menu is not customizable (I think the menu entries are hardcoded in the KDE executables). You can add a new menu section and customize that, but the default is untouchable. At least until KDE 3.5. However you can still delete files by holding the Shift key while calling 'Move to Trash'.
An useful document about Konqueror customization is "
Creating Konqueror Service Menus".