Auto lock ALL text fields and drop downs when sending for E-Signature

I have a form that I am trying to create that will be used by ~100 employees. It has 27 fields, but not will be filled in each time. There are 20 text fields, and 7 drop downs. 

I have figured out how to set the text-fields to 'Read Only' when a box is checked, but all of the Drop-Downs can still be selected and edited by the person who needs to sign it. 

How can I get my form to ONLY have the Signature line as an available option for the signatory? Requiring each employee to go through and manually change each drop down every time it is used is not an option. 

Thanks!