Autonomous drones could easily wipe out all life on earth within this century

 With or without any human help at all.

It all depends upon how they are programmed.

If they are doomsday weapon drones those are set to kill every human on earth in the event of  a nuclear attack upon that nation on earth.

Doomsday weapon drones would automatically kill everyone everywhere in a nuclear war.

This comes partly from the U.S. military philosphy of the 1950s which is "Better dead than Red" which still lives on in our U.S. military in the form of it's drones. But all other nations who have doomsday drones also have their doomsday drones programmed that way too.

How would you stop them?

When they are attacking you would have to create an EMP large enough to destroy the drones and all things electric on earth.

However, once you have done that you have taken human survivors back to Caveman times basically. So, in order to destroy autonomous drones you have to destroy everything electrical in civilization itself (at least in that area of the world.

So, at some point it might be either humans survive or everything electrical survives.

However, I think it is likely we will face this one or more times this century in various countries on earth.

But, once it happens electrical civilization is gone possibly permanently at least where this happens on earth in that specific country or countries.