I guess a starting step would be people with psychiatric ailments or learning disabilities. I knew a couple that had both between them and spent all their time trying desperately to survive while falling for every scheme out there. This was America in the early 90s and they were in their late 40s so surely dead by now, because nobody survives to an old age in the U.S with that kind of weakness.

At what point should the government step in and take away the right of a citizen to enter into contracts?

I had an aunt for whom this would have been (and was) the case, but she was 1 in 50K level of mentally incapable.

What about taking away the ability of a corporation to offer them contracts? I hesitate to call that a "right".

When you take away one side’s ability to enter into certain contracts, you take away the other side’s as well.