When you delete an Outpost, this will completely remove your Outpost from your list. You will not only lose your Outpost, but also all the materials that were used to build it. This process is irreversible, and your Outpost cannot be recovered if you delete it. Deleting an Outpost can be done via the Command Center.
This is not to be confused with stripping your Outpost. This can be done within the Outpost itself and will only remove all allowable blocks, traps, and guards. Stripping an Outpost will allow you to continue building your Outpost.