The roast only works if the room laughs. Be the part of the room that doesn’t.

Cruelty dressed as humour depends entirely on the audience supplying the laugh. Without that it sits in the air as exactly what it is, and the person who said it is the one left exposed. Stopping it does not require an objection or a scene. Simply not laughing removes the fuel.

When a joke lands on someone at their expense, stay quiet and move the subject along. Say something ordinary to the person who was targeted afterward. That is usually enough to shift what the group treats as funny.