I did this a couple of days ago.
You need to know the device name, in my case it was /dev/da0
From a console:
should list available USB devices. If the stick is already mounted (have a look in konqueror system:/media ), unmount it first (in konqueror right click, unmount).
From a console (e.g. Konsole):
# newfs_msdos -L usbstick -F 32 /dev/da0
For more options, type man newfs_msdos
from the console.