Question

recommend ways to deal with a complainer in a workplace

Answer 1

Ask why they are complaining and then try to help them fix whatever the problem is.

Answer 2

You could listen to that person to see if they have ideas of how to fix what they are complaining about. If they don't just tell them I am sorry but I won't worry about it unless you get an idea. DON'T promise to fix it promise to listen to an idea