How do I prevent presenters from changing their poster number?

Unfortunately, there is no way to prevent presenters from doing this.

P.S. In case this didn't fully answer your question or want to see a live demo or ask live Q&A, join our Posters, Pitch, and Art Gallery (Artifact Center) Whova Academy! We host a live session covering the Posters and Artifacts, plus the Certificates tool every Thursday at either 9AM or 1PM Pacific Time.