I was going to say you could go into HSS (High Speed Sync), but it seems none of the Yongnuo's support HSS. 
When using flash, the light falls off very quickly from your subject. The closer you get to your subject, the sooner the light falls off. Try and see what happens when you take the picture closer to the subject (may have to chimp it), and see if you can reduce or eliminate the ambient that way. Move the subject as far forward from background stuff as well. Kind of annoying, but may solve the WB problem if you can.


