This is the source I heard it from:
Sever Force. This removes a Jedi’s ability to use the Force, and was introduced in the Tales of the Jedi comics during the 1990s. Given what we’ve learned of the Force since then, I added the detail that this works by triggering a mass die-off of the subject’s midi-chlorians."
-Daniel Wallace
http://www.starwars.com/news/star-wars-reads-day-special-book-of-sith-endnotes
More like putting a block in the portion of their brain that communicates with the midichlorians in the body, which in turn communicate with the Force.
The BoS explanation is by far the dumbest, though. The mass-killing of midichlorians in the body wouldn't just sever your connection to the Force, it would kill you lmfao.