Simple, just unplug them. Don't cut the wires. The typical airbag connector has a shorting bar that shorts the two wires together to prevent the squib in the airbag from firing. Cut wires are the most dangerous as you can send a charge through one wire and fire the squib. There is potential for current flow. If the wires are cut, splice them together and it is safe as the splice is strong.
The better solution is find an upholstery shop and have them cut and fit a foam block the shape of the airbag module. Take the bag out and put the foam in its place. It is a fairly common thing to do. I've known people to buy seats with blown airbags for cheap, have the material replaced or repaired and put the foam in. Illegal to reinstall in the original vehicle as you tampered with a safety system, and it can be said deceptively as well. But for transplanting into an older car, no different than any other junkyard seat.
The airbag system controls the seat airbags as well. So without the control module and the sensors to match, there is no making the seats transplant with a functional airbag. The tuning that goes into airbag systems (steering wheel, seat, curtain, knee bolster, seatbelt pretensioners, etc) is tuned for the exact crumple zone of the chassis of the original vehicle. The Bronco is so far from anything that has these systems it is more dangerous to try and make them work than to not have them at all.