The 'cleanest' approach I like to take is a complete removal of ALL of JRiver/Media Center's files.
This requires diving down into the innerds of the OS to get all of the pref files etc.
This assumes a degree of knowledge and understanding of what you are doing, because these files are part of the OS and things could go sideways, in a hurry, if you trash the wrong files.
Anyway here is the roadmap to the rest of Media Centers files.
/Applications/Media Center 18.app (File)
/Applications/Media Center 18.app/Contents (Folder)
/Users/XXXX/Documents/JRiver (Folder)
/Users/XXXX/Library/Preferences/JRiver.Media-Center-18.plist (Hidden File?)
/Users/XXXX/Library/Saved Application State/JRiver.Media-Center-18.savedState (Hidden Folder?)
/Users/XXXX/Library/Application Support/JRiver/Media Center 18/Library (Folder)
/Users/XXXX/Music/JRiver Conversion Cache\ (Folder)
~/Library/Application Support/J River/Media Center 18/Settings"
either from a terminal window or the finder after unhiding the ~/Library directory.
Works for me…
JJ