You can't really regulate design - it's too subjective. You could, in theory, start excluding people who's sites don't all validate but, to be honest, that would pretty much exclude all but the most anal of brochure site designers. The fact of the matter is that you can't regulate this industry - you can only educate clients.
What we need is an official, legally-binding checklist/contract that Web design and development companies will need to sign for the client before each job. It would insinuate that the vendor was au fait with Web standards, accessibility,
SEO, etc. Basically a minimum guarantee of quality. If the client felt they were hard-done by, they could have the site audited and, if found badly lacking, they would have legal recourse.