Actually, they can. It's in the Terms & Conditions when you sign up for a Robinhood account. They can prevent buying or selling any issue at any time for any or no reason.
This probably didn't affect your 401k at all, very few funds are going to have GME, AMC or any of the other stocks that most susceptible to this type of trading. The only way there would be any issue is with the blue chip stocks that the hedge funds would have had to liquidate to cover their losses on the shorts.
I'm waiting to start seeing the stories of the kids that put everything they had in GME at $450 that lose everything when it comes crashing back to earth.