Except for reinstalling, the only way to successfully clear the caches is to delete the files manually from your AppData directory.
Delete the solution folder that’s giving you grief in the following locations:
%LOCALAPPDATA%\JetBrains\ReSharper\v7.1\SolutionCaches\%LOCALAPPDATA%\JetBrains\Transient\ReSharperPlatformVsXX\vXX\SolutionCaches\for newer versions.
Note that the version numbers in the paths may be different depending on the ReSharper version that is installed.
The XX in vXX and VsXX represents any number, because there might be multiple folders where the solution cache is stored.