The below is not legal advice and this communication is not intended to create an attorney client relationship between any party. If you believe you have been the victim of defamation slander or retaliation you should consult with a lawyer who is a member of your local jurisdiction.

In a strictly hypothetical situation. If you believe a future hiring body would retaliate against you for acting in compliance with you prior employers directives. I would take the following steps:

Take down the name of the person who may retaliate against you at a later date. This is important because all communication between this individual and the hiring body can be subpoenaed at later date and a spoliation letter will need to be sent. Write a memorandum of the events that you sign and date with times of events. Both the hiring body and the individual who communicated with the hiring body may be liable personally and as a representative of the employer. Depending on your jurisdiction you may also be able to bring claims of slander/liable and defamation against all individuals. All of this advice would highly vary by jurisdiction therefore you should certainly consult with an employment lawyer in your jurisdiction if you believe a retaliatory event has occurred.