Blow up as in BOOM, like the Death Star. Pulverize, or at least break into large, non-contiguous pieces.
And, uh, how much would it cost?
$14.76.
For $16.00, you can take out the moon, too.
The gravitational binding energy of Earth is 2.24e32 Joules. This would require 1.246e18 grams of antimatter. At [thread=239587]$62.5 trillion/gram[/thread], that’s $7.7875e31, ie:
$77,875,000,000,000,000,000,000,000,000,000
But, you know, $14.76 is not all that far off.
But do they have that much in stock at the Antimatter Store?
(They don’t sell it at Wal-mart, do they?)
Try Anti-Walmart.
Dude…President’s Day sale. Duh!
Dangit, Achernar, I was going to post that! But since you’ve beaten me to it, to put that figure into perspective, that would be a cube of water a little more than ten kilometers on a side. Also, standard disclaimers apply about efficiency: You’re almost certainly going to waste a lot of energy blowing shrapnel and various forms of radiation away much faster than needed, and depending on how you apply the antimatter, you might even end up blowing a lot of your antimatter into space before it has a chance to annihilate. If you’re just going to slam an antiasteroid into the planet, and you have to be absolutely sure to get it all in one go, I would probably recommend that you go with between 100 and 10000 times that amount
But we should keep in mind that mass production (no pun intended) usually lowers the cost significantly.
Just out of curiosity, could this be done? Is there enough resources to make that amount? What is the leading antimatter producing company? Also, what positive applications does anitmatter have in our society?
And most importantly, if we made that much antimatter, what the hell would we keep it in until we needed it? 
Tupperware?
Fermilab produces roughly 10[sup]11[/sup] antiprotons per hour. The SLC (at SLAC) makes roughly 10[sup]15[/sup] positrons per hour. To be sure, you could probably squeeze out several more orders of magnitude if all you wanted was sheer production, but you’re still talking ~10[sup]24[/sup] years to get to Achernar’s number. 
1,000,000,000,000,000,000,000,000 years? I’d better get started.
Chronos, do you think a number of smaller anitmatter releases would work better than one big one? Like, say, 10 sites scattered around the globe?
I would actually try to arrange the antimatter around the Earth in as uniform a shell as I could, and then collapse that shell all at once. Or better yet, bury it as deep underground as I can manage, again in an approximation of a uniform shell, and then release the confinement on all the deposits at once. Distributing it uniformly would minimize the amount of antimatter we’d blow off to space unannihilated, and burying it would minimize the energy lost as radiation to space.
Didn’t you see that film in 6th grade science class, “Antimatter and You”? Or were you too busy passing notes?