Is it possible for guns and cars to have an off switch?

Yes, of sorts. Technology is creating a gun that identifies with the owner via a watch, bracelet, under skin chip, or other device. No one else can fire the gun.
I know of no gun that can be turned off by some other entity, otherwise the gun could be hacked, like a computer, that could put lives in danger that need the guns they carry to survive.

^That’s a greater danger i guess..It being hacked!!

A lot of cars already have it. Any car with the OnStar system can be semi-controlled remotely, and the engine locked off.

A remote control for a gun, though, defeats the whole purpose of having a weapon.

Of course. As @zenvelo wrote, cars already have it.

It would be beyond simple to do it for guns, requiring a wifi or bluetooth connection to activate or unlock the firing mechanism.

The problem is the same even with the technological means to do it. The right wing gun nuts would see this as an encroachment on their god given rights to kill people. And the politicians are in the NRA’s pocket, so nothing will ever happen.

Many cars with onboard computers can be controlled without OnStar, using wifi. Really useful for assassinating people and making it look like a car crash.

Even before cars with computers, if the car had electronic ignition, it could be rendered inoperative by a CB radio with a powerful linier transmitter amplifier. Or so I’ve heard.

People will argue that locked guns won’t be easily usable in the spur of the moment when a bad guy wrestles you to the ground or busts into your house.