I think calling them out on it would work. Maybe they’re so used to being pessimistic that they aren’t aware when they’re doing it? Also frequently calling them out will become so frustrating for them that they start thinking of what they are saying.

If this doesn’t stop them from making negative remarks, you need to limit communication with them. Especially in response to anything remotely negative. This will send a clear sign of your lack of tolerance (also you keep your peace)