A photo of the condition of the seatpost and clamp with the clamp bolt tightened would tell us a lot about the situation.

If the slot running up the back of the seat tube closes up completely when you tighten the bolt the clamp is clamping itself not the post. This could occur if you have an undersized seatpost or the seat tube has been reamed out.

Another possibility is that the bolt is too long and is bottoming in the acorn nut. Solutions; shorten the bolt or replace the nut.

Edit: rhenning beat me to it.