The grey x means they closed it themselves. The red x means violation of the terms of service; multiaccounting, being abusive to other members, etc. The other red symbol may mean fair play violation (i,e, engine use), but that symbol is somewhat confusingly also used when an account is closed with no public explanation given.
If you look at the profile of a closed account, there will be a message explaining the reason. For example, look at @Vicky-verma as an example of a fair play closure.
Mmm, if you see the sign it also states the reason...
The red X is for abuse in the forums and stuff like that --- saying big bad words, or saying that down kids should be aborted will get you that icon.
The other one is for fair play violations... sandbagging or c-word. Cherub deliberately sandbagged as if there was no tomorrow.
The last one is when the user closed the account himself, just because he felt like it, and not because he broke some rule.
@pdela is a prime example of the last one, MAY YOU REST IN PEACE GRANDPA.
Wait. Does that mean Diakonia's account was closed because of fair play violations?
https://www.chess.com/member/diakonia