First put the .app from the application file to the trash (any way) Then open the Terminal.app and enter:
rm -rf /Library/Application\ Support/ShadowsocksX-NG-R8
rm -rf ~/Library/Application\ Support/ShadowsocksX-NG-R8
rm -rf ~/Library/LaunchAgents/com.qiuyuzhou.shadowsocksX-NG.http.plist
rm -rf ~/Library/LaunchAgents/com.qiuyuzhou.shadowsocksX-NG.local.plist
rm -rf ~/.ShadowsocksX-NG
rm -rf ~/Library/Preferences/com.qiuyuzhou.ShadowsocksX-NG.plist
rm -rf ~/Library/Caches/com.qiuyuzhou.ShadowsocksX-NG
If the prompt is insufficient, please execute after using sudo