Meddling Mage‘s ability prevents the named spell from being cast. This means that it prevents a player from announcing spells with that name. This ability does not affect permanents in play at all or spells on the stack that have been legally played. If your opponent plays a spell, you cannot put a Meddling Mage into the battlefield before this spell resolves to counter the spell. For example, lets assume that you control a Clone Shell with a Meddling Mage imprinted upon it. If your opponent played a spell you don’t want to see resolve like Red Sun’s Zenith, you can respond by sacrificing the Clone Shell to something like a Rusted Slasher. Doing so will cause the Meddling Mage to enter play and you will choose a card that cannot be played. However even if you choose Red Sun’s Zenith, the Red Sun’s Zenith spell on the stack will not be affected.