How to warn others about a conference-related scam?
On the registration form put, "Housing should be arranged only through [conference's official housing bureau] or directly with your hotel. Use of other services may result in unauthorized charges to your credit card." (Re-word as needed to reflect official housing arrangements.)
Include the same warning, in bold, when you invite speakers. Also, consider asking invited speakers to let you know if they are contacted by third parties.
Warn them like a boss! Warn them like Admiral Akbar:
Actually, I'm at least half serious. Using humor + animation attracts attention, even if it's somewhat garish. Consider making such an image visible people interested in the conference (e.g. on some page of the website).
If this is a bit too much for you, maybe just a still image of the same thing...
How to communicate such scams to a wider academic community?
Report the scam to the relevant authority for your academic community (ACS for Chemistry for instance). They have the audience and impetus to disseminate such information. If they do not, then (IMO) said members ought to push for the society to take proactive measures.
What reasonable measures can I take to prevent scammers from reaching my participants?
Warn your attendees in all marketing materials.
Specify in clear transparent terms the existence of these scammers, how they predate upon attendees, and how attendees may detect them in all marketing materials that is received by attendees, speakers, or partners. This may include: Registration forms, Flyers, Emails, Invitations, the conference website, etc...
The exact wording of this is up to you; but, it must explicitly specify that scammers may try to contact attendees and claim a fraudulent relationship with your conference. As Bob Brown suggested, you may provide a list of all officially partnered entities that attendees may compare to or use. You do not need to push for attendees to register through your partners (as Bob suggested), but you do need to provide the base warning them to take care when registering with non-partners and to perform their due diligence!
Note: Similar proactive precautions taken by other conventions (Comic Con) and Sports Events (Yankees 2016) may also be taken.