How do i deter my friend from recking my relationship.?

My friend Jon dous not understand the gravity of the situation I am in. I barely ever get to enteract with my boyfriend James, He has a full time job. On the days that he comes over to my place. Jon usualy calls. He talks incesantly, and when I say I need to get off the phone to cook dinner for james he won't let me off the phone, and i sometimes have hung up on him, and if i do He just calls back. Jon has said that he thinks I'm pragmatic and loves me. I have told him that i am tentative, and don't think I want a relationship with him. He keeps persisting, and will not mitigate his attention on just me, and find an alternate female. He is becomeing very infamous to me my boyfriend and my parents. How do I get him to stop without being reprehensible