If I were President...
I would have called a press conference to answer the letter.
I would have put Ahmadinejad's invitation to Islam on the ground and stomped on it with both feet.
I would then have had a tank run over it in the Rose Garden one page at a time.
Then I would have peeled it off the treads and sprinkled it with pig lard and feces and set it on fire.
Then I would have put the ashes back in its envelope marked "Return to Sender", along with an engraved invitation to Gitmo.
You forgot - all the while chanting "Allah FUBAR". Might also want to repay the invitation by inviting them to a pig pickin.