If you tell a guy you're not interested in him, and he gets angry with you, it's important to stay calm and avoid escalating the situation. He is more likely to get angry if you've been leading him on or giving him mixed signals. In this case, it's best to be honest with him to avoid misunderstandings.
If the guy does get angry, try to diffuse the situation by remaining calm and positive. This doesn't mean you should try to talk him out of his anger - instead, just try to listen to him and understand what he’s feeling.
It's also important to respect his feelings, even if you disagree with them. If the guy continues to be angry or becomes aggressive, it's best to walk away from the situation or seek help if necessary.