What do i do about my jealous 'friend'??

Ok i started going out with this guy who my friend thinks is hott but we kind of have that relationship where we both usually like the same guys and it's ok for us to go out with them. anyways he's a year older than us and she tought he liked her but he dont cuz she stabbed me in the back (like usual, she's more of a frenemy than a friend) and told him that i liked him cuz she didnt think he liked me. then he suprised her by saying realy? and asking me out. Now she's starting rumors about me and a bunch of guys are saying i'm easy and a slut even though me and adam havent even kissed yet she's sayin we had and i'm not even 15 yet!! how do i just get her out of my life because i know she's not gonna stop. she's so jealous that i'm with adam that she's ruining my life!!! since the lies are coming from someone that's known to be my 'friend' people dont even ask me to make sure they're true and some of my casual friends believe them and i have to tell them it's a lie. wat do i do?