Even if they're being a bit unreasonable, the momentary satisfaction of telling them to F Off is probably not worth the risk of it coming back to bite you some day.
If you don't want to tell them to F Off for this reason, you can contact their line manager, HR or customer service department and ask them to do it for you.
The kind of person to lay into you like this is not going to give you a glowing reference whatever you do.
That is probably true. What I meant is that I would simply break off the conversation at that point. And, yes, if they kept pursuing it, I would either contact their manager if I had a good relationship or have HR tell them to knock it off.
