Like anyone else with a functioning e-mail address, I get multiple pieces of spam in my e-mail every day. And like anyone else with two neurons to rub together, it annoys me. The naive will sometimes reply and say something like "take me off your spam list, vile spammer." While I heartily agree with the sentiment of this, I still think it's stupid. All you're doing in that case is validating that address. It's a tar baby problem.
However, recently I've noticed that some spams include a toll-free phone number. Time on these costs, I am told, between 6 and 8 cents a minute, paid by the owner of the number.
The first time I noticed this, I got my entire office to make multiple-line calls to the number, conferencing it back many times into itself. With a little more work I'm sure we could have gotten that wonderful screeeeeeeeeeeeeeeech of feedback recorded in the voice mailbox.
Lately I've taken to putting on an hour's worth of Israeli trance music, leaving my phone close to the stereo speaker, and going for a walk. The sweet part about this is that these voice mail systems seem to be set up so that, as long as there's a sufficient level of noise, they'll never hang up. This is in effect a Denial of Service attack; while the cost in money may be negligible, it goes a long way to filling whatever these systems use for memory.