Allow delegates to join during the first ballot, as pressure builds. However, after this ballot closes, close the ranks. This helps prevent dirty tricks and forces campaigns to dance with the girl that brought em - us by the way who are already here. We have a system to drop out lesser vote getting candidates. We can use that after this point. If people can keep adding delegates than candidates will, including candidates who are about to be dropped off. We can't in good conscience drop off the lowest vote getter if he can turn around and recruit voters - although we should be highly sceptical of such delegates as they may be duplicates or computer generated - or partisans who want to pick their opponent by voting for the weakest one (remember McGovern in 72 was brought to you by the Nixon Dirty Tricks Machine. Muskie would have nailed Nixon, but was dirty tricked out of it rather deliberately).