....
you can't really.
The building itself could be Wi-Fi disabled, meaning that no signals could enter or exit the building. (done with faraday cages built into the structure and Inhibiting paints on the walls)
BUT, that also means the Matrix is cut off in the building as well, so no real work could be done by the employees in there. Nor could THEY receive E-mails, phone calls, or Matrix traffic of any kind. (which limits their security options as well)
Magical comms are limited in and of themselves, but could be complicated by wards and barriers.
What is the this building supposed to be used for anyway?