This is a schematic where you can put anywhere in your single-player world and on it.

When you on this machine, it'll prevent any tnts that are ignited to destroy near-by blocks.
Note that this can still hurt players and mobs.

FAQS:

Q: How do you install this?
A: You need MCEdit to insert this schematic. Download it here mcedit.net
Open up MCEdit, import this .schematic file into your world. (Place it anywhere near the player)
Once you're done, go back into the game and turn on the device. Now, TNTS won't destroy blocks anymore.

Q: Why didn't this work?!
A: There's 2 reason. Firstly, you are using 1.7. This will work if you're using the 14w07a or above.
Secondly, this device doesn't work when there's two tnts that are ignited at the same time. (Sorry About That)

Download this and enjoy!

Note that this was created before 14w10a and 14w10b snapshots was released...