Most of the suguino or Campy seatpost binder bolts i've seen have a tab under the head of the side without the hex (usually the right or non-drive side of the frame). The frame tab on that side has a slot the tab fits into to prevent rotation while you tighten the other end with a hex wrench.

If your binder bolt and/or frame don't have this arrangement, there are two possible fixes i can think of:

1. Place an external "star washer" or split lock washer under the side that wants to slip. The teeth of the washer should be enough to keep it steady as you tighten the opposite end.
2. Replace the binder bolt with a regular bolt and nut. That way you can put a wrench on each end. It won't be as pretty but it will work.