Can you block someone's mail box by parking in the way of the mail carrier?
It is a common courtesy to not block mail boxes.
According to the U. S. Postal Operations Manual, if a mailbox is
temporarily blocked, "the carrier normally dismounts to make
delivery". If the mailbox continue to be blocked, the Post Master
may withdraw delivery service - but that requires
notifications.
The customer has to have the ability to control access to the
mailbox. If someone is constantly blocking access to your mailbox
so you no longer get mail, that might be resolved by looking at
local ordinances. Some communities might have regulations regarding
that - including parking too close to a driveway, etc.