It doesn't do any harm for your food to go down to zero ... the troops won't be "harmed" by that.  You can go to your Shrine of Odin and click on troops "in the town" and then click on troops you don't want and "dismiss" them.

It my not harm th troops, but t sure plays havoc with trying to upgrade a building or knowledge. 

In your town, go to the shrine of Odin building. Then select the "in town" tab. From there you can dismiss troops.

EDIT, you should only do this is you are trying to have as low as possible food consumption. You only really need to do this if you are banking food. At least the only reason that I do this is because I am banking food and the Loki's chest keeps giving me troops.