A session chairman has to keep things moving. If he lets earlier speakers run over their times, then he cheats later speakers from being able to finish as planned. Years ago at a technical conference I saw Professor Paul Shewmon do it effectively without saying a single word.
In the front of the room, right next to the podium, was a small table with a pitcher of water and glasses. As the speaker neared his allotted time Paul stood up and walked toward the table. Usually this nonverbal signal was sufficient to alert a speaker. Most realized time was running out. They began to summarize and conclude. One speaker kept on going, so Paul walked in front of the table and poured a glass of water.
Another speaker did not even take that hint. Paul then moved almost in front of the podium. He stood there silently, and slowly and calmly drank the entire glass of water. The speaker was appropriately mortified. Without saying a word Paul had eloquently communicated his point. After that demonstration the remaining speakers all managed to finish within their allotted times.
You could use whatever other props are available. If there is a flip chart handy, then you might silently write an invitation to a happy hour or vendor get together after the sessions. You also just could flip open your cell phone, check messages, and then begin to return those calls.