I don't think, though, that this is the problem prospective clients are referring to. Most businesses that don't have websites are unaware of things like spambots. I think what they're worried about are additional customer inquiries, particularly unqualified inquiries.
What I'd tell them is that a site, if properly conceived, should be a work saving tool for them, answering many of the questions that they now deal with by telephone and regular mail. Visitors should get a great many of their questions answered by the site itself.
Beyond that, your prospects may be afraid of unqualified inquiries in particular. This is a tough one. Unqualified inquires can indeed waste a lot of time. SEO generated traffic should be more qualified than most. That is one of the great advantages of SEO... it's targeted.
I haven't searched WW for how to qualify prospects on a site, but it's an interesting question, and I think that may be at the core of what many of your prospects fear, not the spambot thing. (But if they knew about spambots, they'd worry about them too. ;))