You could simply erase the account on the Mac.
On your Mac, go to: Mail / Preferences /Accounts / Account Information.
Select the account and click - (minus sign).
You will no longer receive emails, nor will you be able to send emails with that account.
Alternatively try this:
Erase the info in the field "Incoming Mail Server:"
With respect to the latter, I have never tried this myself, but there is no risk here. If you change your mind you can re-enter the information.