You can name either half of a split card, but not both. If you do so, that half (and both halves, if the split card has fuse) can't be cast. The other half is unaffected.

The named card can be cast again once Nevermore leaves the battlefield.

Spells with the chosen name that somehow happen to already be on the stack when Nevermore enters are not affected by Nevermore's ability.

Although the named card can't be cast, it can still be put onto the battlefield by a spell or ability (if it's a permanent card).

No one can cast spells or activate abilities between the time a card is named and the time that Nevermore's ability starts to work.

If you name one of the faces of a modal double-faced card, players may still play the face you didn't name. (Players can never cast the back face of a transforming double-faced card)