USERNAME typically isn't placed in the Property table; it's initialized when the setup is launched, just as VersionNT, Version9X, AdminUser, ProgramFilesFolder, and many other properties.
You can therefore just add NOUSERNAME to the Property table, and then just save the MSI file (assuming you're in Direct Edit mode).
If you're not in Direct Edit mode, look up "Building a release" in the Developer online help index.