This idea is a spin-off from another idea to educate spam respondents.
In most spam there is a link to a website where you can order the advertised product. When people follow these links, the spam is profitable. Or already when images shown inside the message are downloaded, the spammer earns money
from the advertiser who hired him to reach as many eyeballs as possible.
The ISP should make a list with domains of services spam messages often make a reference to (or get images from). When their DNS is asked to give the IP-address there is first another IP-address given. This is a page made by the provider with the goal of educating the spam respondent not to respond to spam and not to go to the advertised website and turn on spamfilters and so on. Perhaps the images inside a spam message that reside on some server somewhere can be replaced by images educating the spam reader.
But the provider should not be a censor and a link to the IP-address is provided under the educational message from the provider.
There might even be an alternative DNS especially for those who understand the whole spam issue and know how to manually enter the 'uncensored' DNS.
The domains of sites that are advertised in spam can be extracted from the same spam that all the other users at that ISP receive. The moment the message hits the first spamfilter, it is automatically searched for URLs and those are altered in the DNS database of the ISP.
In the end, spammers can only be contacted by e-mail, telephone and fax. Their practices are therefore not completely prevented, but that is not the primary goal. The goal is to educate the users in order to get the number of spam respondents down to zero, making spam not profitable any longer.