an Internet provider that seems to exist for no reason other than sending spam and/or providing spam support services

A website devoted to sending spam.

An ISP or other connected server that exists primarily for the purpose of sending out spam.

Spamhaus is yet another play upon the word "spam": it is used to refer to a site (company) that is not just spam friendly, but actively produces spam. Usually the label is only applied to sites that not only know they are producing spam, but are not doing anything about it. The plural of spamhaus is spamhäuser.