Stumbled across this blog and this handy tool.
This .NET Framework cleanup tool is designed to automatically perform a set of steps to remove selected versions of the .NET Framework from a computer. It will remove files, directories, registry keys and values and Windows Installer product registration information for the .NET Framework. The tool is intended primarily to return your system to a known (relatively clean) state in case you are encountering .NET Framework installation, uninstallation, repair or patching errors so that you can try to install again.
I haven’t tested it yet as I don’t have the need but it’s definitely one for my toolkit!