Hi Steve
If you have to do this via VA02, one at a time, then there is the "Rejection" indicator icon in the header, which can be used to reject remaining qty's on open lines. So you can do all items quite quickly. This assumes you'll apply the same rejection reason to each item.
This is preferable to reducing order qty to match delivered qty, as you then lose the record of what the original customer demand is. Some businesses use the actual ordered qty, unconstrained, as input to future forecasts, and this should use the actual demand, not delivered demand.
Cheers