How do deal with someone copying your very good idea on a presentation?

Answer 1

Don't present it. Write it down, and put your name on it. If you're determined to protect it, copyright it.

This may seem like an extreme response. But it's the only accurate one I can give you.

By definition, your presentation is sharing information with an assembled audience. Once you offer that information to an audience, you have no control over what they do with it.