Make form read-only upon submit

Quick question.  I created a new business object to handle our customer satisfaction survey.  I'm issuing the surveys using webforms via email (we don't want users to have to login to the portal to have to fill them out).  One potential issue here is that users can change their answers potentially forever as long as they have the link handy as far as I can tell.  I think I want to make the form read-only upon submit, but I'm not sure the best way to go about doing that.  Any ideas?

Thanks guys!