I received some great Thank You cards for a speech I gave some time ago. I acknowledged that I received the cards when I saw the senders at another event. Which led to them thanking me again, lol , which was not what I intended to happen.

When, if ever, should we stop the Thank You's?

As you did, one may make informal acknowledgement of a formal, i.e., written, thank-you. That is where it should end.

Frank is correct. Acknowledging a thank you verbally is fine. You do not need to send a thank you for a thank you. However, if your acknowledgement turns in to another nod in your direction for a job well done, take the kudos. But again, you don't have to send a thanks for a thank you note.