This was done by one of our readers:
I've wanted to modify my Ant-Man to close the mouth opening for as long as I have had the figure. I took a razor knife and slid it under the respirator part of the mask. The round piece on the front is not molded to the rest of the head, but plugged into the front. So a razor will separate this portion of the mask fairly easily. Then I painted the mouth area black and wiped some silver paint on top to match the brushed metal look of the rest of the mask, before gluing the respirator back on slightly higher on the helmet. The whole thing took about 15 minutes.
Looks much better than the original!