FAA Turbine engine FCU regulations
Subpart E (Powerplant) of Part 23 (AIRWORTHINESS STANDARDS: NORMAL, UTILITY, ACROBATIC, AND COMMUTER CATEGORY AIRPLANES) has quite a few regulations regarding fuel system operation. In particular, section 23.995, "Fuel Flow".

However, I could not see in there a regulation requiring no fuel to flow in the off position, but given the safety implications of that not occurring, I would fully expect that is a requirement that needs to be demonstrated. 

It appears to exist in Part 33 (AIRWORTHINESS STANDARDS: AIRCRAFT ENGINES), subpart E (Design and Construction; Turbine Aircraft Engines) section 33.75 "Safety Analysis". This states:


  The applicant must analyze the engine, including the control system, to assess the likely consequences of all failures that can reasonably be expected to occur.... The safety analysis must also include, but not be limited to, investigation of the following: (vii) complete inability to shutdown the engine.


I don't expect there are any regulations defining how it needs to be designed to do this - but rather regulations saying what it needs to do, and the way of achieving it is left up to the designer.