If you are their Supervisor, you can view them via the Supervised NPC link at the top of the NPC page, then select the NPCs you want unassigned using the tick boxes then use the drop down boxes at the bottom of the page to select unassign. And then press Execute.
However if you are simply assigned as Operator, there is nothing you can do - just ask the owner to unassign them.
Just adding another question, I am talking to a shuttle service NPC but when i get them to fill out the forum and pay, i get an error sayiing 'error, no pilot NPC presant.