During the registration process, and in the user control panel, provide a checkbox, radio button, or other form control, to let users "opt-out" of receiving emails sent from the admin control panel. Store this boolean value in the 4images_users table.
When the administrator goes to the admin control panel and clicks "Send Email" all the names in the "Select User" list box are highlighted by default. For those users who have opted out of receiving email, simply do not highlight their names. That is:
Current generated HTML:
<option value="User@domain.tld" selected>» User (User@domain.tld)</option>
Would become:
<option value="User@domain.tld">» User (User@domain.tld)</option>
It seems to me that this solution should be pretty simple.