How to make products unpurchasable if foreign currency is blank

We have some products that are only available in certain countries / currencies.

We would like to make the products that don’t have a price in a foreign currency to be unpurchasable.

EG shop default is NZD, and if a product has a NZD price but AUD price is blank, the product should not be purchasable in AUD. However, at the moment, if the AUD price is blank it auto calculates the price from the NZD value at the exchange rate,

i.e. All prices should be fixed prices, no auto calculations – no price means not purchasable.

How do we implement this?
Thanks!