Disable outgoing mail for an account?
There is an option within the preferences of Mail.app.

Go to the "Accounts" tab, and select the email account, and under Account Information, you can set the outgoing mail server to None.

You must also enable Use only this server.

When sending a message from that account, you will now get a message asking you to choose a different outgoing mail server, with the option of cancelling the send.