You may name a land card to prevent it from being played.

Conjurer's Ban only affects cards. A copy of a card (for example, one created by Eye of the Storm isn't a card, so it can be played.

Players may cast spells while Conjurer's Ban is on the stack. Only after all players pass priority does Conjurer's Ban resolve, and that's when you name a card. Once you name a card, that card can't be played in response.

This can't be used as a counterspell. It will have no effect on spells which were on the stack when it was cast, nor on those cast in response to it.