Can i undo the dodo code?

If I'm understanding u correctly, then just closing ur gate at the airport shud mean that dodo code will no longer work an u will get a different dodo code if you open again. Closing the gate wont kick people off ur island, press "-" to do this if they won't leave willingly.

I'm pretty sure everything I just advised is right if I understood what you were asking correctly

Wish I could say the same thing