That is in addition to deciding whether it's even feasible.
Spam is, essentially, broadcasting on the Internet and not a two way communication (though one is desired by the sender for commercial reasons). It is also an inconvenience and a burden to the public. I see no compelling First Amendment or free market arguments against requiring a spam company from buying a federal license and reporting the amount of broadcasting by e-mail that they do and be so taxed.
Look, if spam hadn't been so intrusive and vulgar, no one would care, but any process that abuses the public can expect to be resisted. Spam should get what it deserves. Death by taxation.
How do you propose the government tax offshore spammers?
Don't you think the government already takes in enough of our hard earned money?