Lucky you. If one of these jokers are contacting your clients, it means you've done something right.
I've never heard of them either, but look at it this way: if they're as good as they say they are, they'd be getting enough business from existing clients not to go around bothering you.
Did they use the "you're not marketed in search engines properly" tactic? Or the "Google gives top page ranking to pay-per-click advertisers?" Or my personal favourite, "your clients are losing money with their websites and you should really be more responsible?"
If they start telling you things like that, they usually found you as the result of a search engine lookup. There's another thread on here where I was able to get one of the
SEO companies to admit as such.
In other words, it's a logically flawed argument since it usually means you're doing something right. Just tell them where they can go and move on.