If you engage even one out of ten times, you’re reinforcing it. You can redirect the conversation. Talk to another person, change the subject, completely disengage with them on that specific topic.

You can set expectations privately going in. Set the boundaries. Reiterate them gently but firmly in a general manner. Polite and businesslike when the forbidden topic comes up, cheerful and interested when any other topic comes up. Again, never directly engaging with the forbidden topic.

All this assumes you still want to get along with this person.