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As far as I can tell, you can't return to the Citadel after stealing the Normandy.

I tried it once and it was "locked out" as a destination.

Once the Normandy has been stolen, the Citadel defenses realize Shepard effectively went rogue, so C-Sec and other groups are on the lookout for you. Attempting to head back to the Citadel will result in the Citadel being orbit capable (you can enter the Serpent Nebula, the Widow System, view the Citadel and read the station description and stuff like that), but the station itself is inaccessible. The "dock" option is gone. If you do press the button or click where you'd normally press to "dock", you'll get this beeping sound, to show you can't go back.