I gather that my colleague James has already sent you this answer by email, but for the benefit of anyone else who wants to know:
These instructions assume that you are using an .mdb file to store reports.
Before you start, please note that modifying the SmartAssembly database.mdb file is unsupported. We recommend that you just use the Filter by project and Filter by build filters to ignore test usage reports, instead of following the procedure below.
1. Open %ProgramData%\\Red Gate\\SmartAssembly\\database.mdb in Microsoft Access.
(If you are using Windows XP, open %ProgramFiles%\\Red Gate\\SmartAssembly 6\\database.mdb)
2. When prompted for the password, enter 7f465a1c156d4d57
3. Delete all of the rows in the FeatureReports table.