Nut will not loosen on my bicycle?

I have a Paris Sport road bike from the 70's that I recently acquired. The seat needs adjusted, but for the life of me I cannot get the bolt to loosen to raise/lower the seat. Every time I turn it with a 1/2 size wrench, it takes the nut and the bolt in the same direction. Is there any ways I can just get the nut off so that I can adjust the height of my seat? Thank you!