VBA Digital Signature for macros dropped
 

I have a data entry Excel workbook with Macros that I deploy to my
users with security level high. I sign all the macros for any
spreadsheet and have deployed the certs to all computers
However, I have some users that, manage to somehow strip the
signatures from their document. The macros are password protected so
they can't alter the code. I have no idea as to what they do to cause
the signature to be removed. Somehow they are doing something that
causes the code to change and invalidate the signature even though
they really can't access it.
So is there any method to further protect the code/certificate to
make sure things don't get changed?

Thanks and Regards,
Nals