Maybe host your event in a cave if you can, no cell coverage, no Wi-Fi.

There is a bar like that where I go sometimes, it is in a cave, some people got Wi-Fi from the staff, and you have some reception if you stand near the front door, but it is mostly a network-free zone and it is great.

Another thing we did from time to time at the restaurant is to put all our phones stacked in the middle of the table, anyone who picks up his phone before the end of the meal for any reason pays the bill for everyone. So far, no one did.