Read & sign form

Hello, I am trying to create a form for all my office staff to sign, depending on applicability. So certain forms need to be signed by a certain set of staff, while another may need to be signed by a different set of staff. Currently I have made a usable form in PDF, where the applicability is selected by selecting checkboxes for applicable aircrafts. This reveals the names of relevant staff who are needed to sign the form (I have used rudimentary java scripts for the purpose). For the signatures, I have created PFX signature files for all staff stored on local servers. The form is also stored on the same server for all to review and digitally sign. However, currently, anyone is able to go back and delete anyone's signature. What I want is, once a staff signs a document, that signature should become a read only object. Nobody should be able to go back and delete it. So basically, every signature field should be a single use/edit field. Can this be done? If not in PDF maybe in some other Office file format like Excel? I am attaching the current PDF blank form for understanding.