@Rosemary4
First, you have no evidence that this is the case - just a warning from your block manager to be careful. Which is good an fine. Howver, they may well be a very sensible and nice local couple who are bringing family to the area for the summer.
Did they have reviews? Was there anything susupicisous about their original request - I guess you would not have taken the booking if there was.
I would give them time to respond - many people take up to a week or two if their booking is not immediate.
If by then then don't, contact AirBnB and ask them to make contact with the family and if they can't ask them to cancel on your behalf.
Remember if you cancel, then your calendar will be blocked and you will be unable to let the property to anyone.
You can message AirBnB directly on Twitter - search for AirBnBhelp. Pretty sure they will just ask you to give a bit of time for the guests to respond.
My question would be: they give you the names of the people staying - what does that tell you? Does that make you feel comfortable about the booking?