The method that seemed to work the best for me was to unscrew the bolt, then insert the bolt back into the grommet by about 1/2″ (same hole it came out of) and use the bolt to pry at the head of the plug from the inside until you can pull/pry that ridiculous thing out.
https://www.amazon.com/Splendide-WD2100XC-White-Vented-Washer/dp/B002PZA2UG

