You can only remove imported accounts and external accounts that you imported with private keys or JSON (such as a hardware wallet).
Accounts that you've added using the 'Create Account' button in MetaMask cannot be removed. See below.
See below for a workaround that may let you remove created accounts.
Navigate to My Accounts by clicking on the top-right account identicon:
Here you will see any accounts you've imported; they usually have a label next to the name. Click or tap on the imported account, and the account view will open. Tap or click on the 3 vertical dots menu on the right. This will bring out the dropdown, including the option to Remove account:
Tap on your account name to bring up the account selector screen. It doesn't matter which account you currently have selected.
On this screen, scroll until you find the imported account you want to remove. They should be easily identifiable through the 'Imported' label next to them.
Long press on the account to bring up the option to remove it. It'll immediately disappear from your MetaMask when you confirm.
How do I remove created accounts?
It is currently impossible to remove created accounts -- the ones that appear as 'Account 2' etc. when you click 'Create Account'. If you really want to remove them, you will need to remove MetaMask from your device (Extension or Mobile), reinstall, and then restore using your Secret Recovery Phrase. However, DO NOT do this if you don't have your Secret Recovery Phrase. Without it, you won't be able to get access to your wallet when you reinstall. If you do this, bear in mind:
- This will not delete the account, but just stop it being visible in MetaMask: once created, your address is permanently on the blockchain and cannot be removed.
- When you restore using your Secret Recovery Phrase, MetaMask will automatically re-add previously created accounts if they hold ETH. See here for more information.
- Any imported accounts and/or hardware wallets will be removed at the same time. Make sure you have their private key/seed phrases (or Secret Recovery Phrase if another MetaMask account) backed up, or you won't be able to access them again.