Punters can't leave a positive or negative unless you have confirmed that a booking took place, but comments can be left (where it says Feedback Only) just by them putting a booking form through and waiting for the time to pass. I honestly don't know if this is true if you delete the booking before it's time - we have far more knowledgable ladies here than me when it comes to AW.
If a dodgy client has filled in a booking form, you should be able to leave negative feedback provided you didn't delete it, and you do not have to confirm it to do so. If you take a booking just by phone, text or whatever, there's nothing you can do - particularly bad behaviour belongs on the Warnings board but in terms of feedback you're a bit stuck AFAIK.