There's no such thing as stupid users. Only bad design.
I manage an internal newsletter at EMC, and the easiest way to sign-up users is through a basic Google Docs Form. Because it’s a work newsletter, I only want people signing up with work emails, so I put a note on the form: “Work email address required.”
I still got a lot of signups with gmail, hotmail and yahoo addresses.
Then I made a small change. Instead of labeling the field “email,” I changed it to “work email.” Viola! No more personal email addresses!
Good design is in the details.