how to stop payables invoice from overapplying to a PO distribution?

Hi,

We are on R12.1.

Here is the situation. A std PO with one line one shipment and two distributions. Lets say Dist 1 has 50 amount and dist 2 has 30 amount.

Now, I can match invoice of amount 50 just to PO distribution 2 (i.e. exceeding by 20). How to positively stop this? It just throws a warning but one can overrule it.

Appreciate your help.

Anil