Flash, you said
"On the other hand if he did not want you to lock out the doors, you would need to change everything back. No matter what ops/maintenance says. Its his airplane."
I don't really think you will find many Captains who would go against what a Licenced Engineer has done. If it's broke and it can't be fixed - that's it mate. No matter what the Captain wants, until the aircraft has it's Certificate of Release to Service signed by an engineer it ain't going anywhere. Hence, this is where the MEL steps in, we have a chat with the pilots and come to some agreement based within the limitations allowed and everyone (well nearly everyone) is happy.
As FIMbar says, if the plane is offered for service in accordance with the MEL, the Captain can still refuse to take it with due consideration to other defects the aircraft may have, destination, weather etc. but it certainly isn't his airplane. It's the banks normally.
