Email keeps coming back

So more simply

Go to Settings, Mail, Accounts and delete the Comcast account

Then sign in online to your Comcast email account with a browser in a laptop and delete the emails there.

Empty the Deleted folder too.

Then return to iPhone Settings, Mail, Accounts and add back the Comcast account.

Find a computer.

If we are talking non-Apple email providers go to Settings, Mail, Accounts and delete the account

If it is iCloud mail go to Settings, Mail, Accounts and un-flag the Mail component in iCloud

Then delete them in the web version by signing-in online with a browser in a laptop. Empty the Deleted folder too.

Then return to iPhone Settings, Mail, Accounts and add back the account, or flag on the iCloud for Mail.