The Controller:OutdoorAir references a Controller:MechanicalVentilation object which is always on which references a DesignSpecification:OutdoorAir with OA/person (but this zone has no people) and OA/zone = 9 m3/s. (I should have noticed the Mechanical Ventilation Controller name in the screen shot above.)

In general, the outdoor air flow rate will be the greater of the Controller:Outdoorair Minimum Outdoor Airflow Rate (times Minimum Outdoor Air Schedule) OR the flow requested by Controller:MechanicalVentilation (when it's schedule is active). The schedule in DesignSpecification:OutdoorAir is also applied to any values coming from there which feed into the Controller:MechanicalVentilation. Then add in the economizer and other limits for final OA flow rate. See list of limit checks in order in the Input Output Reference in the introduction to Controller:OutdoorAir.

From this input file, the relevant inputs are:

Controller:OutdoorAir,
Outdoor Air Controller,  !- Name
<snip>
0,                       !- Minimum Outdoor Air Flow Rate {m3/s}
Autosize,                !- Maximum Outdoor Air Flow Rate {m3/s}
<snip>
Always Off Discrete,     !- Minimum Outdoor Air Schedule Name
Always Off Discrete,     !- Minimum Fraction of Outdoor Air Schedule Name
Always On Discrete,      !- Maximum Fraction of Outdoor Air Schedule Name
Controller Mechanical Ventilation 1,  !- Mechanical Ventilation Controller Name

Controller:MechanicalVentilation,
Controller Mechanical Ventilation 1,  !- Name
Always On Discrete,      !- Availability Schedule Name
No,                      !- Demand Controlled Ventilation
ZoneSum,                 !- System Outdoor Air Method
1,                       !- Zone Maximum Outdoor Air Fraction {dimensionless}
Main Zone,               !- Zone 1 Name
OA_Design_Zone,          !- Design Specification Outdoor Air Object Name 1
DataCenter Design Air Dist;  !- Design Specification Zone Air Distribution Object Name 1

DesignSpecification:OutdoorAir,
OA_Design_Zone,          !- Name
Flow/Zone,               !- Outdoor Air Method
0.00,                 !- Outdoor Air Flow per Person {m3/s-person}
,                        !- Outdoor Air Flow per Zone Floor Area {m3/s-m2}
9,                       !- Outdoor Air Flow per Zone {m3/s}
,                        !- Outdoor Air Flow Air Changes per Hour {1/hr}
Always On Discrete;      !- Outdoor Air Schedule Name

A solution that works is as follows:

Add a Maximum Outdoor Air Fraction Schedule to Controller:OutdoorAir which is EMS-controlled (in EMS the condition is defined for setting outdoor air flow to zero). Although this works, this solution is not fully satisfactory because it remains unexplained why EnergyPlus produced a minimum outdoor air flow greater than zero in the first place.

Current economizers cannot provide zero outdoor airflow when the damper is closed, and economizers cannot provide 100% outdoor airflow when the damper is fully open due to uncontrolled damper leakage and perimeter gap leakage. Typical closed economizer damper outdoor airflow without sealed perimeter gap is 10 to 25% and maximum outdoor airflow is 55 to 80%. If the perimeter gap is sealed per US Patent 9,671,125 or patents pending, then the uncontrolled perimeter gap leakage will be 5 to 10% due to damper leakage. Low leakage dampers will have lower leakage, but cannot prevent perimeter gap leakage. Most economizer controllers cannot provide the required minimum outdoor airflow per ASHRAE 62.1 due to economizer actuators not being calibrated per US 10,001,289 and patents pending.