Anytime someone refuses to sign a policy acknowledgement, then you should sign the bottom of it, with a statement that says employee refused to sign this acknowledgement form. I also like to have a 2nd manager witness the document so that there are 2 people that witnessed the employee's refusal to sign.
I added a line to the bottom of my policy acknowledgements that says Refusing to sign this document does not exempt employee from this and all other company policies.