Basically means to tell someone to back-off or leave you alone. Was created from the idea of the phrase "all on my d*ick" or "riding my d*ick", not in a sexual way, but meaning when someone is constantly bothering you or invading themselves into your personal life, among other things. So for someone to say "ride off" is like saying "get off my d*ck", "leave me alone", or "quit bothering me".
Girl #1: Man! My ex has been texting me all day asking me about my new boyfriend!
Girl #2: Are you serious?
Girl #1: yeah! I keep telling him to ride off but he won't leave me alone!
Girl #2: That's a shame...