Well we usually post our mailing address (PMB) as the contact for the whois information along with an email that we can set spam filters in place to ONLY allow emails from specific registrars to get through to us.

One thing that youc an do t stop being pestered by these companies though is every time one of them calls you, write a templated email off to their domain registrar as well as whois that way they should, in theory have their domain and hosting stopped as it is against whois rules to use the information contained in a DNS record to contact individuals and companies for sales purposes

I know we have done that a few times and after about 5 days the offending companies no longer have a web presence

Regards,

Lee