It it my sincere hope that the interesting little history behind this use of the word SPAM is not forgotten. This was a facinating time in computing, at the dawn of newer, sprier operating systems, and the Internet. And, as is so often the case with language, the history of a word is the history of people, ideas, values, feelings, social philosophies, and on and on.
NOTE: Spoiler: The key fact is that "SPAM", in relation to Internet email, did NOT originally refer to commercial bulk email!
If this piques your interest, read on!
A collective response quickly evolved; a notice went around to "SPAM the bastard," and hundreds (or thousands) of bulky return emails were dispatched post-haste to the sender of the despised commercial email. All you really had to do was attach a huge file to a single email.
Under earlier, less robust versions of UNIX, the email daemon—an ever-present background process that woke up and went to work under certain circumstances—would stash return email on the root spindle. With the massive volume of SPAM, the offending party's root spindle would fill to 100% and crash (and hopefully burn) their system.
Because this action had a perfect parallel in the Monty Python's "SPAM sketch," this return email was called "SPAM," like the male chorus drowning out the folks in the Python sketch. Again: Notice that "SPAM" originally refered to this return email, NOT the original, offending commercial bulk sending.
However, the noun "spam" and the verb "to spam" had become so cozy and familiar that it was kept, although, now that there was no retributional return email, it mutated to refer to the original commercial email.