loosen a stuck screw
The screws are likely rusted in place.

Drill the head completely off so you can remove the aluminum parts. Then try the following:


generous application of penetrating oil, followed by a 24 hour wait. Put vise grips on the shaft (pliers will not work) and use a hammer on the vise grips. You want a sudden movement here, not a steady turn
heat the part and then use the vise grips again. And by "heat" I mean glowing dull-red. Grab it with vise grips while still glowing.
the above may simply twist off the shaft, but that's step 3 anyway. Grind it flush with the rest of the metal, heat it up bright red, preferably orange, and let it cool completely. Do NOT quench - heating softens the metal, rapid cooling hardens it. Drill and tap a new hole. If there's too much corrosion in there, drill a bigger hole. If it's really nasty corrosion, go all the way through and use a bolt, and if it's bad enough that the bolt won't work, get a new grill.