Hmmm, this doesn't sound good!
I would suggest you contact Airbnb via twitter and let them know the details of the properties. A lot of hosts list their properties on a number of platforms other than Airbnb and also concurrently. Airbnb would need to investigate further and If they find evidence that this host is indeed advertising on Airbnb and then encouraging the guests to bypass Airbnb and book directly then I’m sure they will decide appropriate sanctions.
If guests search for accommodation on the Airbnb platform then decide to bypass Airbnb policy as well which requires all transactions, for both the host and guest protection to take place on their platform, then as harsh as it might sound, the guest have no one to blame but themselves. It is a small fee we as hosts or guests pay to Airbnb for their services which include protection in situations like this; unfortunately some people prefer to take unnecessary risk for the sake of saving a few pounds/ dollars etc.
If you know the properties in question, then you can also go directly to the listings and flag them for the attention of Airbnb.
Click on the link for contact options, Twitter would probably be your best option for this sort of query and normally guarantees a quick acknowledgement or response - Contact Airbnb.
BW, Ama