The relationship between Spam, the packaged meat, and Spam, the Internet phenomenon is quite dubious. Most Americans intuitively, viscerally associate "Spam" with "no nutritive or aesthetic value." Although it may be true that Spamming on the Internet had no aesthetic value, the link between the meat and the Internet problem lies in a sketch by the British comedy group, Monty Python.
The scene is a British eatery. The waitress is recommending various dishes, that all contain Spam, in large proportion to other ingredients. A group of drinking Vikings in the back chant "SPAM, SPAM, SPAM, SPAM, SPAM, SPAM, . . ." in a crescendo, to the point that it becomes impossible to carry out a conversation. Hence, the meaning of Spam in the Internet context: multiple postings reduce the signal-to-noise ratio of a newsgroup to the point of making it useless for its intended purpose.
The full script of the sketch can be found here:
The Monty Python SPAM sketch.
If you have a Real-Audio Plug-In for your browser you can listen to it: SPAM.