It only takes one experience with a speaker telling off-color and offensive jokes to your association audience to realize it can happen at any time. Best to address it up front, with any speaker.
Here's an excellent form by Jeff Hurt that covers a range of potential speaker issues, including selling from the podium. It's on a blog post that has LOTS of great info for anyone who hires speakers (e.g., sample A/V requirements form, A/V recording authorization form, speaker travel/lodging form.)
Jeff explains, "A Speaker’s Guidelines Form (includes information about using non-sexist and non-discriminatory language, the use of intellectual property, as well as a non-sales from the podium) This is an important form that protects you, the conference organizer and lets the speaker know you expect professionalism."
Thanks, Jeff. Great info!