Get rid of my pothead friend?

So i've known this dude for 5 years, we are (or should i say were) best friends for a while. but after his first year of college he became a serious pothead, i have smoked many a time with him but he now smokes like he needs to, almost like he's addicted or something (ik its not addictive im just saying). and the only time he calls me now is to smoke, but i promised myself to never smoke again so i can focus only on school work. he found out and was calling me soft and lame behind my back. i want to curse him out and put him in his place, but hes just so immature, ive seriously grown out of this friendship. what do you guys think i should say to him to get him to not hang out with me anymore (a subtle yet totally understanding process)?