Cell service is very different from WiFi. Jamming cell service is illegal, blocking WiFi AFAIK is not illegal.
This all depends on what you do and how you do it.
A building with a metal structure that effectively blocks RF signals, would affect wifi and cell to varying degrees. This kind of passive block is legal and happens all the time, both intentionally and just as a result of construction methods.
RF jamming transmits a strong signal containing noise, it is tuned for the band they want to disrupt and can easily be wifi or cell. This is absolutely illegal. It's also pretty easy to find simply because a jamming signal is still a signal and can be tracked just as any other signal can. In fact, the stronger the jamming signal the easier to locate.
Now devices that hijack cell communication are a little different, They impersonate a cell tower so the cell phone registers with them, because they offer the better signal. At this point they can be legitimate cell relays (smart type) and essentially act as a proxy to establish your connection to a cell tower. Used like this, with no attempt to interfere or monitor the communications, they are legal and easy to get. But this same device could simply be used to register your phone and not pass on the connection. Now it's illegal. This would also be much easier to explain as faulty or misconfigured equipment.
The last type is a cell repeater, really just an rf repeater on cell frequencies. To get these to work, and they can be legal, is a pain because both antennas are using the same rf band. For example the phone side antenna would be inside the steel building at one end, and the tower side (uplink) would be outside the building as far away as possible. Turn up the power, inject some noise, and now if just an RF jammer.
Because all these use a signal, to disrupt a site the device would need to be in a fixed location, and this makes it easy to locate. Report it to the FCC and they may get around to tracking the interference. Tell all the local Hams and they might just turn it into a game and find the source. Then you can give details to the FCC and they are a lot more likely to do something. Or, once you know who is responsible for the rf source, and you can show damages, get a lawyer.