Removing Certain Roles from Add User Drop Down

I am using a Role Editor and have a few roles setup in my WordPress install. Example:

Administrator (by default this one is already there)
AccountExec
Customers
Demo
None

Is it possible to either remove or hide the Author, Editor, and Contributor options in that drop down by adding a custom function in the theme’s functions.php file? If so, what would that look like?

Thanks for any help!
Tony