The air pump has a clutch that engages the air pump when the car is first started up to pump air in to the emmisions system. You are indeed correct it is attached to the belt system. Disconnecting the plug will disengage the air pump clutch and so it will be disengaged and should not engage the pump unless the clutch itself is trashed.
2005 320 CDI
2006.5 VW Jetta TDI
1991 560SEL (179000 Sold)
1972 280SEL 4.5 ('The Lead Sled' 320000 miles when sold.)
1972 220D (225000 when sold)