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 really want to no longer see them in your wallet, you could restore your wallet (do not do this unless you have your Secret Recovery Phrase recorded somewhere, otherwise you will lose access to all of your accounts). Any accounts that have a zero ETH balance will not be automatically re-added during this process, meaning you'll no longer see them.
MetaMask is software that you can delete at any time. The accounts that you manage in MetaMask are permanent, and, once created, will be around for as long as Ethereum is. See also: What's the difference between a wallet and an account?
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.
