You can purchase short-range blockers or signal jammers. A Google search can find several vendors, or try Jammer. Similarly, receivers that detect cell phone or WiFi hosts are available.
You need to confirm that your legal jurisdiction allows the use of those devices.
Assuming that you are in the US electronic cell phone jammers are illegal pretty much everywhere. What you can do to paint the walls of the room with paint which has copper flakes in it. This will create a barrier which will prevent any wireless signal from working within the room.
Go keep in mind that if your have a repair tech that wants to use their cell phone to get information on a problem while working on your servers, that tech won’t be able to.