In b2b you never say no, always have an option for what they're asking and charge more for it.

This was a somewhat recent revelation for me, and it totally works.

"Well, we don't really want to work on this or agree with the approach, but pay us double our normal rate, and we'll do whatever you want."

Typically that will end the conversation, and in the rare case they do move forward, being paid handsomely is really nice