Close the game, and if you are using ASM, close that as well. Navigate to your game.ini and put this down the bottom of the text file:

ConfigOverrideItemCraftingCosts=(ItemClassString="PrimalItemStructure_PMH_MailBox_C",BaseCraftingResourceRequirements=((ResourceItemTypeString="PrimalItemStructure_TechPlot2RC_C",BaseResourceRequirement=2.0,bCraftingRequireExactResourceType=true)))

Make sure it is the folder <yourserver>\ShooterGame\Saved\Config\WindowsServer\
Inside of the game.ini file
I use ASM to host, so this is from my perspective. It will not remove the item from the list, but at the same time, will disable the ability to craft the mailbox.