Marianna,
Sadly, the problem that inexperienced/incompetent clients don't know what they want initially or dishonest clients expand their requirements after an Upwork contract is in place, what you describe is not uncommon (based on the number of posts we see here about this problem from other freelancers).
In the first case, doing a project on an hourly rather than fixed price basis can help align the amount of work you eventually do for a client and what they actually pay you.
In both the first and second case, it is really important to clearly define the deliverables on any project. This would include details about just what you will do, but also what you will not do in order to complete the contract. (I doubt many inexperienced freelancers bother with this second bit, but they often ought to.)
Inexperienced clients can/might/often will agree that their post-contract add-on requirements are not in line with the original scope of the contract. You'll just have to stand your ground on "more work, more pay" with the dishonest ones..
Either way, once a contract is in place the client has a permanent ability to leave both private and public feedback for you. We have been told on this message board that the private feedback is significantly more important in calculating a freelancer's Job Success Score.
I think I'd just leave this sort of contract open but idle, in the hope the client might just go away and not close the contract on their own (which you can do yourself long after they've gone away and the client might not even be paying any attention any more and won't leave feedback for you). Others here will likely have an opinion on that approach, which I have never actually had to do because I don't have to drag things out with my clients.
Good luck!
