Connect your Stamps.com account to Calcurates to display USPS real-time rates at your store's checkout
Create a USPS shipping option and enable certain USPS services (USPS Priority Mail, USPS Ground Advantage, etc.)
Add Shipping Segment as a preset condition for your future Restriction Rule.
Add a Shipping Rule with a "Hide" action applied to USPS services using your Segment.
Optionally you may add a restrictive message displayed to your customers at checkout in the Rule's settings.