How do you make a guy who is extremely into you, even though he knows you aren't single, to bug off?

Be kind yk? tell him have some respect for your relationship and if he can't then cut him off completely

First advice I'd say tell him the truth ... That you're not interested. That you are in a relationship and don't want him interfering. Tell him you will file a restraining order against him. Or block him from everything, and ignore him.