If you can still load up the solution, try the following:
File -> Source Control -> Change Source Control and then unbind and/or disconnect all projects and the solution.
This should remove all bindings from the solution and project files. (After this you can switch the SCC provider in Tools -> Options -> Source Control -> Plug-in Selection).
If you can no longer load the solution, try the following:
In file explorer, delete the .suo file, start a notepad instance as the administrator, load the .sln file in notepad and remove the entire GlobalSection that refers to (TeamFoundationVersionControl).
GlobalSection(TeamFoundationVersionControl) = preSolution
xxx
xxx
xxx
xxx
EndGlobalSection
Try loading the solution now and you should be fine.
Source: http://stackoverflow.com/questions/358951/how-can-i-completely-remove-tfs-bindings
