How to publish proof of something if the academic level is lower than expected and cannot publish to a journal or a conference?
I will echo @jakebeal's skepticism about your results themselves - I would suggest having a long, serious, and honest discussion with a professional mentor about the novelty and importance of what you've found. This is important both for presenting your ideas, and deciding where they should end up - I certainly have thoughts that I know don't "rate" $TopJournal.

That being said, there's nothing about being an undergraduate student that would present you from publishing in a solid journal. For example, as an undergraduate working in the field you described, I published in several solid journals. There's nothing that would stop a paper from being published just because the author is an undergraduate.

You've said you've contacted several journals and conferences, and they've refused - have they said why? Have you spoken to your mentors to make sure your paper is actually submission ready - the process is non-trivial, and important even if your findings are genuinely important. What you should do next, if you're genuinely interested in moving your finding forward, is to talk to people in the field, make sure the idea is ready to publish and that you've got a solid notion both of its importance and how to "sell" the concept to an editor, and then proceed from there.