Cell phones in jails & prisons
... a bit of a problem ...

Various approaches have been tried to deal with this, and it looks to me like there are a couple that would actually work. One of which might be a bit cumbersome ... shield the whole place. Wrap the whole place in copper screen (and ground it well). Naahhh. Let's blow that one off. It might be appropriate for a small cafe where you don't want people to sit and blab on their phones. Then the "no cell phones" would be a statement of fact rather than a request or demand.

The other thing would be having a cell site right there, so (by the nature of the cell phone system) any and all phones would be working off that site. Of course that would entail a multitude of equipment, although if the desire is to have no cellphones work at all, that would simplify matters a bit — all phones would connect to a site with no connectivity at all except to the front office. Then if any legitimate calls need to be made, they can be patched through manually to a landline.