You can't turn down watts but you can use common sense.
Like Mike C said, you will should able to hear the tell tale signs of speakers running out of steam before they cross the point of no return.
It's not that difficult.
If it starts to sound funky, harsh, distorted or in any way simply not right, turn it down immediately. Something is complaining. It could be either the speakers, the amp or your ears. It doesn't matter. Something is trying to tell you it ain't happy.