I don't like a guy but I wont straight up tell him to leave me alone
Why not just tell him to leave you alone?
I scare him and tell him straight and candidly to stay away from me and I treat him like a shit.
You shouldn't lead people on. If you don't like someone you should tell them so that they have time to deal with it too.
Why should I tell him the truth? just because.
It seems like you enjoy leading this guy on, which is wrong. You really should tell him the truth about how you feel.