Due to the nature of the blockchain, users cannot permanently delete an account on the Ethereum network. If you no longer want to use MetaMask, simply uninstall.
As we don't store any user data whatsoever, there is no account to delete; your ETH addresses are permanent on the blockchain and can simply be forgotten or abandoned if you like.
If you'd like to remove MetaMask from your browser, just right-click the browser extension icon and click "Remove from Chrome" (this process will be similar for all web browsers).
On mobile, simply remove the app as you would any other, and according to how you normally would on your device.