(U.S.) 14 CFR 91.123 (b) states:
Except in an emergency, no person may operate an aircraft contrary to an ATC instruction in an area in which air traffic control is exercised.
FAR 91.123(b) was written for a reason and it has likely been violated before (on purpose or by accident).
The question specifically:
Is a pilot receiving ATC VFR service (e.g. traffic advisories/flight following) subject to 91.123 (b) when operating in class G airspace (uncontrolled) or does this regulation only apply in controlled airspace? (considering the wording of the regulation [in pertinent part]: in an area in which air traffic control is exercised