This happens to me all the time.
I don't think the guest is trying to trick you, I don't think he's blackmailing you, and I definitely don't think he's a jerk.
It's just that many guests arent' super experienced on the AirBnB site, have trouble booking the dates they want and keep fiddling around until they get to where they can send you a message. (happens to be an inquiry for different dates).
This happens really a lot for me, and I just explain it to the guests.
BTW I've raised this time and again with AirBnB that 'Entered wrong dates' should be a reason for declining a stay, because it happens so often and it's the only reason why I ever decline guests. (EDIT: Wow! Just read above that “guest requested different dates” is now available!!! Excellent! )
However with all the recent changes in policy for AirBnB I'm now very worried about declining anyone for any reason. I prefer to ask guests to cancel their booking inquiry, or if it's just an inquiry and not a booking request then I prefer to just let it sit there without declining: I'm not confident enough that AirBnB won't at some point punish hosts for declining, even based on past declining of months or years ago.
Anyway I digress: in this case all you need to do is say "sorry, we have an X day minimum stay. Please consider me again for any longer future stay."
And then don't decline, just in case.