Back in November I put out a plea for aggregators to stop polling for discontinued scraped RSS channels. I coupled this with an Apache redirection to a custom RSS channel containing a single item with a discontinued message. The url of the item in the RSS channel was timestamped so that it always looked like a new item to the offending aggregators. My intention was to get the attention of the aggregator users who could then get their sysadmins to remove the channels (I'd tried and failed to contact them, most being inside .edu domains).
Well, it looked like it worked - and pretty quickly too. I haven't had a hit on any of the discontinued channels since the day I made the change.