I was looking through the Canvas Guides, and it doesn't look like this is possible. You can select all people in a class or select specific people. However, you can't choose everyone and then delete certain people from that list...from what I can tell. I was looking at this Guide for reference: How do I compose and send a message?. I wasn't able to find a suggestion similar to what you were asking, so you might want to create a new Feature Idea by going to this link: Canvas Feature Ideas.
Also, keep in mind that the messaging system is not really e-mailing. It's messaging. Students (well, any user for that matter) can choose whether or not they want to receive e-mails via the Messaging/Conversations area of Canvas. They could, potentially, choose not to receive any e-mails at all and receive all communications via text message, for example. Also, users can choose the frequency of those e-mails, text messages, etc. (Right away, Daily, Weekly, Not at all).
Hope this information is helpful to you!