I used to get about 30 a week. Don't know if they still have it now, but the reply envelop was postage paid. I took them all and filled them with folded newspaper or scraps and mailed them back.
As I had mentioned before, they appear to get all the details from the OpenSRS Whois DB.
In the UK, they could be prosecuted under Data Protection Laws - I shall be contacting our Data Protection Commisioner about this, to see what can be done.