Ha ha, I think nobody up to now has understood your question in the right way (excuse me if I'm wrong :)). I assume you meant something like: "don't go on with the subject you/we are speaking about". You could say: "Laisse tomber"; for example, because the topic is not important or interesting. It's a bit like "Forget it". or "Restons-en là"; in this case, in most situations, you ask for not to go on with the topic because you're getting angry... There are other expressions with a similar meaning as for example: "Changeons de sujet" (Let's change the topic) or simply: "Arrête, s'il te plait" (Please, stop it), but this last is rather agressive, as you can guess.

Hum it's depends if you want leave your friend or not. If you want leave your friend you can say : " Je dois te laisser."

you can say: " je dois quitter mon ami"

quitte-le

laisser