Open Gmail
Click the gear icon
Select See all settings
Go to the Accounts and Import tab
Under Send mail as, find the email address
Click delete or remove next to the email address
Confirm the removal
If needed, go to Check mail from other accounts
Find the linked mail ID
Click delete or remove next to it
Confirm the removal
If the mail ID is your Google account email, change the primary email in your Google Account settings or create a new account
Sign out and sign back in to confirm the changes