If you have to use a creeper that runs up to and attacks a player you need to refer to something like InfinteReaper suggested.
Else you could try something like this (pardon me if I'm wrong, doing this off the top of my head)
execute @e[type=Creeper,r=#] ~ ~ ~ summon Fireball ~ ~ ~ {Motion:[0.0,-1.0,0.0],direction:[0.0,-1.0,0.0],ExplosionPower:#}
Replace the #'s with the number you desire for max detection radius from command block and explosion power respectively to blow blocks up depending on blast resistance, and all you have to do is add a block doing
kill @e[type=Creeper,r=#]
after the previous command succeeds.

A fix that comes to my mind right away would be setting blocks in the area you want the creeper to explode on a loop so that it replaces the block if it is destroyed, and if you want to be really technical you could tag the items in that area and destroy them as well.

Then, set up another command block that will set the creeper's fuse time very high and its explosion radius to zero if it's outside the area you want it to explode. And when it comes inside the area you want it to explode, change its fuse time and explosion radius back to normal.

Or, you could probably find a way to tag the creepers when they are in the radius of the area you want them to explode, and as soon as they blow up you could remove the specific blocks via command blocks that activate afterward.

I know I'm not offering you any actual commands to accomplish this or anything, it would be somewhat complicated to pull off. But I'm just brainstorming. Hopefully it helped.

http://mcstacker.bimbimma.com/...

I heared about something like that Sethbling made...
But I think it didn't work (anymore).
For my map, I used an explosion and a fill command to only fill cobble.

Otherwise you may be able to execute this fill command right before it explodes. Would use it'S "Fuse:" NBT tag