Publication 17 says to write NRA in the box, see page 25: http://www.irs.gov/pub/irs-pdf/p17.pdf
And you are right that you do not need to apply for an ITIN if you do not claim an exemption and do not elect to treat the NRA spouse as resident alien.
Update for your 2014 taxes:
https://ttlc.intuit.com/replies/4600744
