The 8-1 ruling closes the courthouse door to inmates who claim that they are being subjected to double punishment for a single crime.

Several states, including Florida, adopted these forced-confinement laws to cope with the problem of dangerous sex criminals whose prison terms were about to expire.

Under the measures, state officials can keep custody of inmates who they say are dangerous and mentally imbalanced. If a jury agrees, the inmates are confined for extended treatment. Typically, these inmates never leave prison grounds, although they may be transferred to a separate building.

While popular, these laws were seen as vulnerable to legal challenge, since the Constitution bars the government from subjecting persons to double punishment for the same crime. This is known as the ban on double jeopardy. The Constitution also forbids ex post facto laws, or after-the-fact punishment.

But the Supreme Court cleared away most of these constitutional concerns four years ago. Ruling in a Kansas case, the justices on a 5-4 vote said that forced confinement can be seen in principle as a kind of civil detention, not criminal punishment.

Then, the Supreme Court left the door slightly open for confined individuals to show that the state treatment program was a sham.

Forced confinement could be 'so punitive in purpose or effect' that it would be, in reality, continued imprisonment, wrote Justice Clarence Thomas in the 1997 opinion. For example, an inmate who is kept behind bars in a prison-like facility surrounded by barbed wire could claim that he is still in prison, not in a treatment center. On January 16, however, the court closed off such legal challenges.

A state law 'found to be civil cannot be deemed punitive as applied to a single individual,' Justice Sandra Day O'Connor wrote for the court.

Only Justice John Paul Stevens dissented.

If the facts show an inmate is held in prison, not in a treatment center, he should be allowed to go to court to argue that his rights are being violated, Stevens said.