There is no such thing as a passive jammer. You can shield your building as much as you want, intentionally or not, and it may block cell phone signals &/or other signals, or not, but if you're not transmitting a signal, it's not a jammer.

Any device that transmits a signal that interferes with licensed radio services is a jammer, if intentional, or is simply an interference source, if unintentional, but it is illegal either way.