Short story probably from pre-1985, probably originally published in Analog. A group builds a machine that will be able to broadcast telepathic messages to everyone in the world. They realize that it will be easy for anyone to build a similar machine leading to chaos (these are not mind-control signals, but being spammed telepathically would still be bad). They therefore send the first and last telepathic broadcast, which contains easy directions for building a device that will block receipt of any telepathic messages.
I checked the list of titles for Analog's "Probability Zero" stories (flash fiction, generally humorous) and one possibility is "Mass Communication" by Jay Kay Klein, August 1985. Sounds promising. (confirmed not to be the right story).
This USENET post is talking about the same story I am, though
Since broadcast telepathy is the technology you want anyone else to have even less than nukes (unblockable propaganda... spam...) someone puts together a transmitter, transmits the plans for the telepathy-blocker, and that's the end of that technology.