Inquries and spam on vacation rental sites

Shortly after our launch we found out that spammers and hackers are everywhere. They pose as renters, they post as owners, or they just come to the site to harvest phone numbers.
I think in today’s age it is impossible to create software that will filter these spammers. But here are few tips:
1) Once you have a visitor from Nigeria, block his IP.
2) If a visitor emails to 126 owners with the same text, something is wrong.
3) If owner is checking his property from always changing IPs, that should raise a flag
4) Connected from Russia and has 3 properties in Paris? I don't think so.

All 1-4 can be legimate users, but this is something that should be checked manually. I think it is a responsibility to the vacation rental portal site owner to prevent bad situations. It only takes 1 bad transaction to kill the portal.

We are, at http://VacationRentalsListed.com take it very serious. We have someone who monitor all inquires and postings. We have built in a special feature for admin to do it.