I assume it won't let you delete it because the reference is already complete? What reason do you have for believing it may be bad? SOPHAS only requires three recommendations (and most schools only require 2-3), so you could try calling SOPHAS and requesting that they remove it. I really can't guarantee that they will do it, though. Have you already submitted your application?
EDIT: Sorry, this only applies if you are MPH. I don't really know what you should do if you are MHA.