+1’ing the importance of having this issue fixed here too!
Think the only proper fix would be what is suggested on this feature request (go vote all of you who want to see a solution!) 
Toggle Last.fm Scrobbling by Service – i.e. having a toggle for using last.fm on Roon per service & local files.
On the bare minimum, Roon should’ve added a note to the last.fm configuration screen warning about this! I just messed a full year of last.fm stats due to this issue since I had no idea it’s a thing, eventhough this has been flagged here several years ago…