Had an internal status page that everyone in the office would refresh often; the bosses decided that was a great place to put "message of the day" texts for whatever notices they liked. Of course they started writing 500 word messages every day, din't delete anything, and people stopped refreshing the status page, never having read the "management wisdom" that was being splashed around liberally over the actual important information contained on that page.
What worked was to put a 1 day time limit on management messages, and a 200 word limit. If they wanted to keep a message up two days they had to type it in again the next morning.
Also, we put a random "fortune cookie" text on the status page that changed at every reload, under the "management message" area. That encouraged people to actually read the words under the status numbers.
Everybody but management had a very positive response to running the "management message" bit through `cowsay`; too. Dunno if the effect would have lasted, but consider it for things that are important.