The general rules:

You can’t prevent your clients from hearing about or or Craigslist.

You can’t prevent others from pitching your client with a really low price.

You can’t disparage those low prices directly, nor the people who offer them. And it rarely works to raise fear, uncertainty and doubt about these ‘fly-by-nights.’

You can’t prevent a client from trying out one of these ultra-cheap sources.

You can’t collapse to your knees and match their prices. Nor should you even lower your prices as a concession.

Best bet. Actually, the only practical move:  Let them go and try.

(I’m paraphrasing now . . . )

Client: “I found I can get this a lot cheaper.”

You:  “That’s interesting. How much cheaper are they talking about?”

Client:  Mentions a painfully low number. Like the loose change between the seats in your car.

You: “Gee, that’s an irresistible price alright.

“That’s like one of your customers saying he can buy one of your [whatever he sells] for nine cents. I don’t know how to deliver your kind of work for that. But if this source works out for you, you’ve found a heck of a bargain there. 

Bottom line, if the client can indeed be satisfied by a $3/hour designer, they weren’t going to be a client for long anyway.

If the client tries the ‘kid working out of his parents’ attic’ and gets back a piece of junk, he will call you again.

Oh, and don’t ask how that cheapo worked out.  Let him save face.