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How do I improve the app's behavior in this use-case?
In the sign up page, the user is prompted to choose his time zone. After signing up, the user gains access one page where he can use the app's features, including adjusting his time zone.
The thing is, once the user publishes an object in the app, his time zone should be locked for further changes.
How can I encourage the user to be aware and remember this behavior?
The general principle is to bring attention to the user at the point of need, so that they are aware and can take action when it is necessary to do so.
In your case, if you only allow the user to set this value once and it can't change, then there are two logical places where you might want to prompt them with this information, depending on how important it is.
The first place is when they are about to make a selection, because this is where they will have to think about what is the best choice. And the second place is when they are about to confirm and complete this action, because this is the point of no return where they can't make any changes afterwards (if that's the case). You may decide that it is important enough to make them think twice, or just once because it is where most of the users will have made up their mind.
Good design principles that support a clear alignment between user activities and system status/feedback would suggest that any permanent or destructive action require the user to actively opt or confirm these decisions. A lock of the timezone doesn't necessarily mean that they can't change this, but if it is difficult to do so then they will need to be aware of the consequences of the action.
If it's not already included, add a confirmation screen at the end of the sign-up process where a user can review and edit their details before creating their account. This means they can see which time zone they've selected, and change it if necessary (for example if they made a mistake during sign-up).
Once they're logged in, it gets more complex, and really depends on the wider functionality of the app.
How integral is "publishing an object" to the app? Will users do this often? If this is the main function, then it's unlikely that the user is going to edit their time zone settings first. They're going to want to cut straight to publishing. And if you've added an additional confirmation step, it's less likely that they need to make edits once they're logged in. In that instance, I would remove the ability to change the time zone once logged-in altogether.
If, however, "publishing an object" is a rare occurrence, or something that might be done months after signing up, it would make more sense to allow a time zone change to a logged-in user. Maybe they've had time to move away since they first registered.
I think more context is needed in terms of the purpose of the app, before deciding which settings should be available. Will they be using the app for a brief time? Or for years? (which would leave more likelihood for moving locations and time zones). What is the reason for "locking" the time zone? Does it really need to happen? Maybe you should make this "lock" clear during the "publishing an object" stage, since this is the action that will directly affect the functionality.
