How about I tell my ex to fuck off?
Why do you want to tell your ex to fuck off?
My ex is boring and has nothing to offer me.
You are not in a relationship with your ex anymore so it is time to move on. You are being angry and mean to someone for no reason. If your ex is boring then it is good that she is your ex.
I don't want to lose her as a friend.
Well then don't tell her to fuck off if you want her to remain friends, it is not okay to be rude to people that you still care about. Especially if that reason is for jealousy or spite.