Leaderboard

You tell them that you are a web developer, not an SEO consultant. Go hire an SEO consultant. If they want you to do it, there's an extra charge - get them to pay you a monthly retainer to manage their SEO needs. If they don't like the results they can cancel anytime. This falls into the bracket of managing customer expectations. Also putting people off getting you do silly work. If you don't want to do it, put your price up so they don't ask you to do it... or you're happy to do it because you're getting paid well.