To uninstall Mono on OS X utilize the uninstallMono.sh script. Where is it? How do you get it?
Utilize Finder to browse to /Library/Receipts. View the package contents of Contents/Resources/MonoFramework.pkg.
Drag a copy of uninstallMono.sh to the desktop. Open a Terminal window. Then cd desktop and run: sudo ./uninstallMono.sh and enter your password.
Mono on Mac OS X is then uninstalled (bin, Frameworks, etc.)