In other cases the client is a company client, and you might not be in a position to say no.
Nothing stopping you from explaining your opinion as an expert on the tech, and if it's extra time (==money) also communicating that. If that is ignored and you're getting paid for your time, that's the best you can do.
This is usually what happens, though in my case my boss (Lead Dev) is already telling them what isn't possible and what doesn't make sense. Some clients just insist for months until we finally give them what they want and they see we're not talking out of our rears. But yeah I understand all that, it's just silly to me to waste time and money doing things that will not be useful for the end-product. I guess I'll be numb to the silliness eventually...