I tried putting the name on my Microsoft account as "FUCK MICROSOFT" but it said "Your name can't include special characters." changing it to "Fuck Microsoft" gave the same issue. Only when I made up a fake name did it accept it.
Not sure if this is a bug or if it blocks swear words. If anyone knows why it does this please leave a comment.
I'd suggest that it's better to use actual fake identities for privacy anyway, rather than some juvenile attempt to get back at them. Why paint a target on yourself like that?