Just pause awkwardly for a second, long enough to be felt but not long enough for them to start talking again, and bring up something unrelated
It’ll hit them with the feeling of social rejection, but without the confrontation or giving them anything to latch onto. Nothing to get offended about or argue against, there’s nothing to react to there
It might take longer, but it’s not a request to stop - it’s training them to not bring it up. It’ll make them uncomfortable to talk about it - even if they force themselves it’ll be uncomfortable for them
(Unless they’re high on the spectrum, in which case direct is better all around)