This is from the "how to send" part of this website:
There IS NO USPS regulation concerning "Any Soldier", however there is a DOD Postal Manual (DOD 4525.6-M) Issue #22093 dated 9 January 2003, which states, "a. Mail addressed to "Any Service Member," or similar wording such as "Any Soldier," "Sailor," "Airman," or "Marine"; "Military Mail", etc., is prohibited. Mail must be addressed to an individual or job title such as "Commander," "Commanding Officer," etc."
I would suggest (a) trying another clerk (b) trying another post office. The person you spoke to is confused.
The one time I had a clerk who said "I don't think you can do this," I just said "I checked, and you cannot address items JUST to any soldier, but this one is going to a specific person." She said "OK" and that was the end of it.
Good luck!