Chronic interrupters threaten not only your presentations but your image in the office. After repeated occurrences, the situation will become either comical or embarrassing to the rest of the staff, and the content or import of what you say will be lost. Try to deal with this in private first, even accepting some of the responsibility. If that elicits an apology, leave well enough alone. Denials should be met with examples and a blunter request. Attacks, on the other hand, should be met with a sharp rebuke and even a veiled threat. If the problem crops up again, respond with all the ammo you can muster. Failing to be forceful at that point will guarantee that the problem becomes worse.
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