I had the exact same thing happen in my fourth siege, except the representative and his guards were already in negotiating. Everyone outside in the caravan was butchered but all the goblins were killed except ONE at the very edge of the road, loitering in the carnage and the trade goods. I told my squads to move out and go get him...they milled around the barracks. I figured "to heck with it" and let all the dwarves loose to deal with it. They all ran away from the goblin. Stalemate.
Then, a brave little war dog attacked it and got crunched. Along comes another war dog and takes the sucker out. The cowardly dwarves rushed in to grab all the free goodies, sparing no time for the good dog, who wandered off, somehow looking pleased with himself. Then the three humans remaining left and wandered sadly off home...I don't know how ASCII icons can look bereft and sad, but they did.
In my third siege, I had open doors on the outside with trap corridors with locked doors on the inside. After the first few got wiped out, the goblins got smart and loitered around outside. So I opened one of the inside doors, they rushed towards the door, I locked it again - and they kept going inside. I feel like that's kind of cheating, though, since I'm winning the siege and not the dwarves, by careful pausing and relocking when the dwarves are closer than the goblins.