In case anyone else having the same question visited here, please visit the official guide.
The following is quoted from the original guide which was updated at 2017-4-14.
Uninstall Visual Studio for Mac App
Step 1. Locate Visual Studio.app in the /Applications directory and drag it to the Trash Can. Alternatively, right-click and select Move
Step 2. Remove all traces of Visual Studio for Mac
Execute the following commands in your terminal:
sudo rm -rf "/Applications/Visual Studio.app"
rm -rf ~/Library/Caches/VisualStudio
rm -rf ~/Library/Preferences/VisualStudio
rm -rf "~/Library/Preferences/Visual Studio"
rm -rf ~/Library/Logs/VisualStudio
rm -rf ~/Library/VisualLodStudio
sudo rm -rf /Developer/MonoDroid
rm -rf ~/Library/MonoAndroid
sudo pkgutil --forget com.xamarin.android.pkg
sudo rm -rf /Library/Frameworks/Xamarin.Android.framework
rm -rf ~/Library/MonoTouch
sudo rm -rf /Library/Frameworks/Xamarin.iOS.framework
sudo rm -rf /Developer/MonoTouch
sudo pkgutil --forget com.xamarin.monotouch.pkg
sudo pkgutil --forget com.xamarin.xamarin-ios-build-host.pkg
sudo rm -rf /Library/Frameworks/Xamarin.Mac.framework
rm -rf ~/Library/Xamarin.Mac
Uninstall Workbooks and Inspector
Uninstall the Visual Studio Installer
rm -rf ~/Library/Caches/XamarinInstaller/
rm -rf ~/Library/Caches/VisualStudioInstaller/
rm -rf ~/Library/Logs/XamarinInstaller/
rm -rf ~/Library/Logs/VisualStudioInstaller/
rm -rf ~/Library/Preferences/Xamarin/
rm -rf "~/Library/Preferences/Visual Studio/"
Uninstall Mono SDK (MDK)
There are other applications outside of Xamarin which also use Mono, such as Unity. Be sure that there are no other dependencies on
Mono before uninstalling it.
sudo rm -rf /Library/Frameworks/Mono.framework
sudo pkgutil --forget com.xamarin.mono-MDK.pkg
Other apps with name prefix Xamarin
The official guide didn't mention it, but I found there were other apps named Xamarin.xxxx in my library directory. I simply moved them to trash.