I am working in a hosted environment on a mac. I have been told by support that I cannot remove the decimals in my thank you letters. E.G - the amount in my thank you letter is $1000.00. Our team likes to use $1000 and leave off the decimals unless they are necessary.
This is one of those things that is related to your Windows/Word configuration. You can easily use some of the info about merge fields here to change this http://iits.haverford.edu/documents/mail-merges-formatting-numbers-in-a-merge-field/
I also do this really easily by doing a find and replace on .00 -- first thing I do after a merge.