Don't allow PO Box shipping addresses

Hi,

Yes. You can edit the field "address_street" via the menu Display>Custom fields and use the "regular expression check" setting to forbid the use of PO Boxes.
For example, such regex:

^(?i)(?!p\.?o\.?\sbox|post\soffice).*$

Hi,

There is no option for that for now as you can't have fields for one type of address and not for the other type.
You would have to do the check with custom javascript instead if you wanted to do that, but you would have to ask a developer to do that for you in that case.