When exporting an FBX file with ver 6, it exports materials that do not exits anymore

However, when i export using the v7 FBX, it exports just fine.
I need the 6 version for some compatibility with other softwares, but it keeps materials that i deleted or replaced, it makes it very annoying to deal with the materials on the exported version, is there any way to make him remember what materials are not relevant anymore when exporting in FBX v6?