Well, im sure you know, the stalk comes out by just pulling on them. They are just sort of pressed in.
So, I would guess to try and get a screw down the middle, so you can have something to pull against. Other than that, drill out as much as you can, and try to pick the rest of it out of the turn signal switch. Or just replace the turn signal switch all together.
Fortunately, the stalk and the switch are pretty cheap for our trucks...