What you possibly have holding the grilles in place are metal things called speed clips. They bite into the plastic and hold 'em thight. Nice things if you never have to remove anything. If you do need to get them off it's 5 minutes for each of them....
I would personally opt for the route that HB Chris pointed out; gently destroy the clips; a dremel will do just fine & quick if you can fit it in there.
Most of the times the metal clips will snap in half when you put a side (wire) cutter on to them, but that does require you to wiggle them a bit up (towards the end of the plastic stem) to be able to get one half of the cutter's beak under them - A dremel is faster as you can just start without the wiggling work.
I know of one other neat trick, although it does take a while: The starlocks have teeth that grip into the plastic stem.
it you pry a small screwdriver in the hole between respective 2 teeth, and lever the screwdriver up, then you can force one corner of the tooth up, and the corner of the adjacent tooth down. If you do this in all 6 holes, you've essentially created a thread, and it will now gradually spin off when you grip it with pliers and rotate it. Benefit is that you can reuse that starlock now over and over. I also use this trick BEFORE installing a starlock; it will allow them to be unscrewed easier, if ever needed. On some of the alu trim parts i've actually resorted to just cutting an M4, M5 or M6 thread on them e.g. the rear trunklid badge and the alu trim on the center console around the window switches.
