In either TFS 2005 or TFS 2008, you can delete the test results published against a build (How to: Publish Test Results).  However, discovering this feature may be difficult, and I didn’t find it on MSDN in a quick search of the docs.

What you’ll need to do is to bring up the build details (aka the build report) in Visual Studio.  Then expand the test results, right click on the link to the Test Run and choose Delete.