You certainly don't have to set up the USPS from ShipStation option, but there is not a way to remove it from your Carrier settings. It will always be present for you to choose to set up later, in case you do decide you want to use USPS services in your account.
If you have already gone through the setup steps but you don't want those USPS services appearing in your Services drop-down menu when configuring your shipments, just disable all of the services for USPS in your carrier settings. When no services are enabled, the USPS options will not be present in the Services menu any longer.
To do this, go to Settings > Shipping > Carriers (or Carriers & Fulfillment, depending on which country your account is based in). Then click the Services link next to the USPS option:
Then, deselect all the services in the list and click Update.
Hope this helps!