How do I remove an alias (email) from my account?

Go to https://www.icloud.com ; sign in and go to the Mail page. Click the cogwheel icon at bottom left and choose Preferences. Select the Account tab. You will see a list of your main address and your aliases: select the one concerned and click 'Delete'. (See image, right). Deletion is permanent - you cannot reinstate it (you can if you choose 'Disable alias' instead).

Note that together with your main @icloud.com address you may also have @me.com and possibly @mac.com versions (with the same text before the '@') depending on how long you've been a member. These are tied to the main @icloud.com address and cannot be deleted. (They are not strictly speaking 'email aliases', which are additional addresses, though they are sometimes referred to, confusingly, as 'aliases'.)