Letter
Well, here is what you do hopefuly you kept some old rejection that were sent to you.
Then what you do is look at the various styles they used,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,, copy what you like then read it over to make shure it makes sense.
In this area, i would be very shure before i sent any thing to a future employer, even if at this time you have no interest in working for them.
May be you schould just send them a card thanking them for their interest, but at this time you have decided to move in a different direction at this time, mite not want to burn any bridges in this day and age.