A common technique for killing woody plants is to cut them down and then paint full strength herbicide concentrate (of about any type) on the outer ring -- cambium layer (just below the bark) of the cut trunk. The herbicide will stop new growth there. However, the tree could sprout new shoots from anywhere along the roots. No way to determine if it is prone to that except by anecdotal stories, such as ... I know of more than one instance where live oak was cut and the roots sprouted a veritable LAWN of new live oak shoots. Eventually, through mowing and herbicide apps. they succumbed, but in each case it took a period of 3 years or so. There is a lot of energy stored in roots.

Maybe, maybe not. You have to remember that there is probably just as much living plant material UNDER the ground as there is showing ABOVE the ground. Especially as these plants in question are several years old.

As Yard said in his/her message, herbicide is also my weapon of choice.

I do the same. Cut if off and then straight RoundUp on the new wound. If it sprouts I let it grow for a bit then give it the same.