BASIS : want to disable the pricing conditions in VA02

Hi,

When we create or change the Sales Order from VA01/VA02 with refrenced to contract always.

It copies the pricing conditions from Contract.

We are using PR00 pricing conditions.

In Sales order it is Write mode. But we need to disable that, so no one will make changes in the PR00 conditions.

I need to do this by Authorization. As soon as I give change Autho for SO document then any one can change PR00 also.

Pls suggest.

Points will be rewarded.

Thanks