Hi, obviously I cannot see exactly what you have got, but assuming it is working as an air vent, I would not attempt to chisel away at it, could be very messy and give you bigger problem. You say it sticks out approx two thirds of an inch, in that case I would get some 1"x 1" timber or such like and make a frame around it, then fit a close fitting plastic sliding vent ( buy these any diy shop they come in various sizes) . You can paint the timber white or whatever colour suits. If you are tiling around the vent area you can tile first then fit new vent cover. To me it would seam a shame to replace with plastic what sounds like a good solid vent.
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