I'm new at all this but it seems we should not be forced to make the plaintiffs' case for them.
"I don't recall and I have no records indicating that transaction."
Now, whether or not the court believes you is a different question. And to my mind that's the real crux of the issue. If you got sued and deposed within three months after your last payment the court is not likely to believe that you don't recall. But a year later? Five years later? That's much more of a judgment call. Not everyone has the memory of an elephant.
Nor am I suggesting that you lie and pretend that you don't remember when you do. My point is that a lawsuit involves not only facts but strategic decision making whose results cannot always be accurately predicted in advance. Do you have to make the plaintiff's case for them? No. But think carefully
about what you say and do because some judges will not take it kindly if they think you are being "slick".
The question is not, "Can I refuse ZZZ". Of course you can refuse. You can refuse to ever answer the complaint to begin with. You can refuse anything. The question is, "Will this decision put me in a better place than I was before." And sometimes that's a gamble.