You can use the second preference in the collector category and set it to None(Preference Manager > Revise >Collector > Include dependent CAD/Dynamic Documents). However, this will also exclude collection of any part of assembly related to the drawing.
OR you can try putting the drawing formats in a library and provide only read and download access to users.In this way even if the collector collects the format, the format doesn’t get changed owing to no revise permissions.
Hope that helps.
Regards
~Syed