What are some Islamic instructions on who you should discuss Islam with and when to end it?

Does Qur'an invite us to go and preach everyone or it invites us to go and have endless debates or there are certain limits? Are there any points where the debate should end? Does Qur'an or the narrations offer any guidelines for this? Or there are no guidelines and we should keep the debate/discussion ongoing no matter if the person keeps denying?