Hard as it is to believe, this 1970 English comedy sketch is the origin.

It is amazing that it caught on YEARS later because in 1970 there was no Internet – but a few years later, Newsgroups debuted and the term just took off and lived on.

http://en.wikipedia.org/wiki/Spam_(Monty_Python)

http://youtube.com/watch?v=wZ7YedEopp4

Actually it's not so amazing. Everybody loves Monty Python.