I just bought a Craftsman lawn mower and there are two pieced of plastic that stop the blade from turning. I've never seen this before and don't know how to remove them. They are attached to under the deck somehow.

My suggestion would be get make, Model #, might also need serial #. All mowers have them. Look for an aluminum plate, approx. 3"×4". On it you'll find that info. I'd take a pic of it. With that info, use the internet, check owners manual web sites. Enter the information requested, all you'll need is right there.

There are 2 screws that are holding them on if it the lawn mower I'm thinking of they are under an easily removed

I would say lookup their website