You may not hinder a manipulative person from saying demeaning things to you, but you can refuse to take their word seriously.
You should be on the lookout for people who attempt to make you feel below par in your work performance, in beauty, or in any other way that would hurt or shame the average person.
Any person who is genuine in trying to forge a friendship with you will not in any way utter something that makes you feel awful about yourself or that puts you down.
Psychopaths use this strategy of making you feel inadequate or of little worth so that when they subsequently ask you to do something that favors them, you find yourself wishing to accomplish it in order to prove to them that you are not worthless after all.
If it is something you are not comfortable doing, such a person is likely to put some pressure on you; sometimes stressing how cowardly you are if you do not do it. Learn to dismiss such negative sentiments.
Anyone who puts you down or blackmails you does not have your welfare at heart and is out to manipulate and misuse you.