Have you removed Roon from the Applications folder? If not, open Finder and click on the iMac under Locations, then navigate to macOS, followed by Applications. Right-click on the Roon icon, and select Move to Bin.
Next, go to the Finder Go menu, and select Library. Now delete the Roon and RAAT folders.
This should have completely removed Roon. Kindly note that this is irreversible, so be certain you want to take this action. It may be necessary to restart the iMac.