Add this between the [start:users] and [stop:users] loop:
[if:admin]
<form action="[send_url]" method="post">
<input type="hidden" name="to[]" value="[user_id]" />
<input type="hidden" name="subject" value="Official Warning" />
<input type="hidden" name="message" value="You have been warned." />
<input type="submit" name="submitForm" value="Warn Member" />
</form>
[endif:admin]
Feel free to change the subject/text or even allow moderators to access it.