Treat everyone nicely, even when they reject you. You never know when your gracious response may help you in the future.
If you act poorly or inappropriately, people will remember that. It may feel good to “tell them off” in the moment, but it builds ill will towards your future self.
On the other end of the spectrum, if you act kindly and with grace, they may think to help you in the future. They might be rejecting your idea right now because they don’t agree with it, but that doesn’t mean they don’t like you. Don’t take rejection personally. Instead, thank them for their time and try to get feedback from them as to why they said no. Now, you can use that feedback to improve your idea (or the communication of your idea) in the present, or gain a better understanding on how to present to that person in the future (what do they value?).