My understanding is that the company calendar is only used for production planning and that purchasing uses a default 5 day work week.

First off, is that correct?
Secondly if so, is there any way to prevent MRP planning purchase order deliveries for certain dates? We have shut down weeks where the company is closed and would like to prevent MRP planning for deliveries during those periods.

you can refer the U8745A (lead time), it explain the plan and the LT in very very detail.

I was under impression that Purchase Lead Times use 7 days calendar. But you are right. It is using 5 days calendar by default. I checked it with some simulations.

Best Regards,
Shekhar

This is possible. You can make those weeks as "unavailable' in calender working hours and then generate moments. Also check/delete the records in the Delivery moments for both Shipment and Delivery based. Then it will not generate MRP planning purchase order deliveries for certain dates.

Another place is linking calender in Buy-From BP so that any holidays at supplier facility are taken into consideration while planning purchase orders.

Best Regards,
Shekhar