Remove these directories:

/Applications/Xamarin Studio.app
/Developer/MonoTouch
/Developer/MonoAndroid
/Library/Frameworks/Mono.framework
/Library/Frameworks/Xamarin.Mac.framework
/Library/Frameworks/Xamarin.Android.framework

Locations in your user directory:

~/Library/Caches/Xamarin
~/Library/Caches/XamarinStudio-4.0
~/Library/Developer/Xamarin
~/Library/Developer/XamarinStudio
~/Library/Logs/Xamarin
~/Library/Logs/XamarinStudio-4.0
~/Library/Preferences/Xamarin
~/Library/Preferences/XamarinStudio-4.0
~/Library/Xamarin
~/Library/Xamarin.Mac
~/Library/MonoTouch
~/Library/MonoAndroid
~/Library/XamarinStudio-4.0

That’s all there is afaik.