Fantasy suggestion:
Walk right in and flip the room lights on and off while announcing: Intermission is over! Everyone please take your seats. Our class begins now. Dr. Jack@ss, please take a seat if you intend to take my class.
I just typed a long reply and it disappeared.
I would actually do what geonerd suggested for real, aside from the last line.
Talk to your Chair and give her specifics.
Ask a collegue to come by and watch you lecture for feedback and they can see his behaviour (don't announce to the class or him that you are brining someone along to evaluate your teaching).
Have a plan where you can start teaching without needing the computer. Ask your students for a quick summary of what you did last class, an answer to a question you gave them at the end of the previous class, start a discussion. you can also start using the whiteboard and transfer to the computer later. If Dr. Jack@ass has written on the whiteboard, erase part of it (it doesn't seem like his lectures are going overtime, so you wouldn't be interupting his teaching). If he's still there when class starts, start talking as if he's not there. Don't let him get you flustered as it seems he gets a kick out of that. If he's talking to students in your class, tell them to class has started and to stop talking.
As for the student who stood up and said he was switching classes, tell him you wish him the best of luck and if he's transferring out of your class, he can leave now.
