How can I delete one of the "from" choices for my outbound email account?

One of my email providers is discontinuiing email support operations and I'm in the process of directing all correspondence to one of my other vendors. When I send emails, the "from" field has a drop down menu to choose which email the correspondence is sent from. Currently I have three choices there but I would like to delete the discontinued choice so I don't have to constantly check that Mail is not sending out the discontinued return address. Anyone have an idea how to do that short of deleting the whole email account before I need to?

Thanks.