I teach 6th ELA so I realize three years makes a difference. Teaching in a private school as opposed to a public school also makes a difference. But I do have to say, you are dealing with this MUCH better than I would be. Not only did they not have a speech finished in six periods, but they wanted to circumvent your consequences, by choosing their own topic (which they could have done in the first place) and then write it in one period or less? I would be furious if my whole class came to me with that statement/proposal. Sounds like it was an intentional sabotogue - or did I misunderstand?
Since they had six periods, I would have told them that they one extra day (with penalty) to complete what they had started and present - finished or not. If they still had nothing, they would present a speech on the topic I would choose - and they would not be fun topics.
But then again - I am old and mean. :x So what did you do?