Hello Robie, and welcome back to Just Answer.
If you take some deep breaths and keep your mouth busy by chewing gum or sucking on a mint, etc., you may be able to prevent yawning. Also, try to keep your mind busy/alert by taking notes while the speaker is talking. However, yawning is a natural reflex and a way for your body to increase your oxygen level, so it's often difficult to stop it. I'm sure the speaker understands that people need to yawn. It's not necessarily a symptom of boredom with the activity (listening to a speech), but could be a symptom of not having gotten enough sleep and/or just a need to take in more oxygen. As long as you politely cover your mouth and not make any noise when yawning, it should not be considered offensive when listening to speeches, etc.
I hope this helped.
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