Bend the spring fingers in one at a time a small amount.

You can also place a block of wood on the plug, hold the free end, then strike the middle of the block.
A 2x4 about 12 inches long works well.

Yep that's what I ended up doing, bending the fingers, placing it in the box, and then bending them out again. Pain in the butt but it worked. Thanks for the help.

I witnessed the removal of some that are 50lbs each and about 3/4" thick. They had the tabs just like a regular knockout but a sledge hammer is used to gain access.

"They had the tabs just like a regular knockout but a sledge hammer is used to gain access."

He is talking about plugs used to fill unused holes when the KO has already been removed.

I know brick.. sorry to sidetrack the knockout topic reminded me of seeing the huge boxes and huge knockouts and thought I would share.

I dated a knockout once...

I dated a knockout once... was he/she too tight?