Card Text: Emrakul, the Aeons Torn can’t be countered.
When you cast Emrakul, take an extra turn after this one.
Flying, protection from colored spells, annihilator 6
When Emrakul is put into a graveyard from anywhere, its owner shuffles his or her graveyard into his or her library.
So is there a card that CAN destroy Emrakul while she's still on the stack?
No. You only destroy things on the battlefield. You can counter things on the stack, but Emrakul can’t be countered.
Here are some examples of things you CAN do to Emrakul:
Exile it with Oblivion Ring. (When Oblivion Ring targets, it isn’t a spell anymore. Emrakul doesn’t have protection from triggered abilities of permanents.)
Destroy it with Executioner’s Capsule, or Avatar of Woe. (Again, Emrakul’s protection doesn’t protect it from abilities of permanents.)
Destroy it with Supreme Verdict, or put it on the bottom of its owner’s library with Terminus. (Emrakul’s protection only stops it from being targeted and damaged by colored spells, not destroyed or removed some other way.)
Return it to it’s owner’s hand with Venser, Shaper Savant. (This works whether Emrakul is a spell or a creature. Much like Oblivion Ring, by the time Venser targets Emrakul, Venser isn’t a spell anymore and Emrakul’s protection doesn’t stop abilities of permanents. Also, Venser returns spells to hand without countering them, getting around Emrakul’s can’t-be-countered ability.)
Steal it with Gather Specimens. (That’s just nasty.)
I won a 4 way with four mana. Played Quicksilver Amulet, turn after that played Emrakul, and on the turn I get to take after playing him I drew Jace then attacked the two remaining people in the game for 15 each.
Can I block Emrakul with a colorless creature that has a colored enchantment on it?
Yes. Permanents on the battlefield aren’t spells. Emrakul only has protection from colored spells, not creatures.
(In the same vein, you can target Emrakul with Oblivion Ring or Executioner’s Capsule, because they aren’t spells once they’re on the battlefield. None of Emrakul’s abilities protect it from activated or triggered abilities of permanents, or from being blocked or damaged in combat.)