You can just do a rushed diplomacy check, that way the only way they can interrupt you is if they readied an action before you started talking. You take a -10 penalty, but hey, if you're a diplomancer you can just eat the penalty, and even if you're not serious all you need is hostile -> unfriendly to stop a fight, or unfriendly -> indifferent to get them to listen fairly or barter. So even rushing you only need to roll a 30 to stop a fight or a 25 to get merchant to stop being a butthead, easy peasy.
Edit: gack! Ninja!