How to deal with negative co-workers? who demeans on others.?

Question

How to deal with negative co-workers? who demeans on others.?

Answer 1

When the opportunity arises, and, it will, tell them directly that their behavior is inappropriate and self defeating.

And, when they respond with more negativity, tell them that they are exhibiting the very behavior which will doom them in the long term.

Answer 2

Smack them upside the head.

Or give enough detail to explain what you mean.  That would be better.