How can I delete my work email from my iPhone?

I recently retired and need to delete my work email. My employer apparently has deleted me, for I keep getting annoying pop ups saying my password is incorrect. I can hardly use my phone because the pop ups happen every 10 seconds or so, no matter what I’m doing. The usual instructions to go to mailbox, choose the mailbox, hit delete, do not work . There is no DELETE MAILBOX choice.