Due to the nature of the blockchain, users cannot permanently delete an account on the Ethereum network. You will simply need to uninstall MetaMask.
We don't store any user data whatsoever, so there is no account to delete because 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 tab and click "Remove from Chrome" (this process will be similar for all web browsers). On mobile simply remove the app.