I'm a bad libertarian, so bad that many libertarians wouldn't give me the honor the title...I use it because, despite my badness it's still probably the best fit of my ideology which can be summed up as the belief that liberty is a pearl beyond price.
So...I think the primary role of a just state is the preservation and protection of liberty. In our modern age we look upon the state as the greatest threat to liberty, and yes in light of the 20th century this is probably the case. However a problem I find with many libertarians is their failure to recognize that private actors can be just as great a threat to liberty as the state. In the 18th century the East Indian Company, the Slave Cartels, and many the owner of a private plantation and mine were probably a greater threat to liberty than just about any state government on the earth at that time. Also one easily forgets that one of the ideals behind the existence of a strong monarch is that such a monarch would protect the liberty of his subject from the many lesser tyrants of the world, English history is full of examples of strong kings who did just that.
Ok...back to this topic. I feel that the tyranny of the corporation is a legitimate fear in the absence of state over-site...BUT there is little doubt in my mind we live in a second era of Robber Barons, and this in spite of the massive size and scope of our federal government. Indeed it appears that the federal government, to some extent, has largely enabled the Robber Barons of our era to achieve their inflated state.
That leaves us in the state of damned if we do damned if we don't. I believe the solution is keeping all institutions, public and private, limited in both size a scope.
As to specifics of the op, complete freedom to contract will lead to slavery pure and simple. After being captured in war the number 1 source of slaves in history has been people contracting away their freedom, either directly or as the result of defaulting on the agreement. For that reason alone I'm opposed to it.