Are you on WP 8.1 or 10?
Do you mean a single email or an email account?
WP8.1 Single email
Open the account tap the email you want to delete and a pull down screen will pop up with DELETE.
WP8.1 E-MAIL ACCOUNT.
Go to Settings
Go to E-mail+accounts.
Tap and hold on the account and a pull down will pop up with DELETE.
If you want to keep the account, because you also have contacts or use the calendar then after deleting the account add it again but disable the email sync.
If you wanted to know how to do it in W10 please join the site and reply on this post or make a new post with more details what you want.
Good luck.