Be careful not to deny employment on grounds of being mean since there is no objective standard. Try to maintain a positive and upbeat work environment. This can bring out the 'best' in a mean person. Help them to recognize they are perceived as being mean. Note we did not say are mean but perceived as mean.
Awareness is the first step to improvement.
I use a Personal Commitment Form. If they don't want to improve, consider their appropriateness for their position. Their meanness, and all its defensiveness and negativity, might outweigh the value they deliver. Overlook the small things. We all have faults.