did you somewhere read that one can stop PO creation just by removing the calendar assignment?
Which error message did you expect?
This KBA explains that the factory calendar is not checked: 2280534 - Factory Calendar not considered when determining a default delivery date
Use MM17 to assign a material status that does not allow any procurement to all materials in your US plants. Make sure that nobody schedules or runs MRP otherwise you may end with countless requisitions.
In general you should remove the authorization for the organizations until you are ready to go live.