I am very sorry if you are having to deal with a client who is behaving badly.
But ultimately, only Yahor can fix this problem.
Upwork can not monitor every word spoken by clients and everything they do.
YOU, as the freelancer, need to manage how work goes with clients. The place to put a STOP to clients asking for free work is when it very first starts.
Do NOT do any free work for a fixed-price contract. Do NOT do ANY work outside of the original agreement.
Don't allow it.
Don't do it.
REMEMBER: If a client asks you to work for free, then the client is violating Upwork ToS.
Clients who ask freelancers to work for free can be reported to Upwork. There is a "asking for free work" reporting line in the "Flag as inappropriate" tool found in the top right hand corner of every job posting screen.
Clients who ask freelancers to work for free may be removed from the platform.
If you agreed to draw a picture of 4 cats, and the client asks for a 5th cat for free, then don't do it.
If you agreed to write a 1000 word essay, and the client asks for 5 more words for free, then don't do it.
But you don't need to say "no."
You can say:
"Yes, that is a good idea. I would be happy to do that. If you will close the current fixed-price contract and start an hourly contract, then I can do any of these tasks using the hourly contract. Alternatively, you can release payment for the current milestone, and add a new milestone of $150 so that I can draw a 5th cat."
If you have a fixed-price contract, do ONLY what is specified in the agreement. No more.
If the client asks for work OUTSIDE of that agreement, then he needs to set up a new contract.
If a client doesn't understand how fixed-price contracts work, then you can ONLY work with her using an hourly contract.
If you are already working using an hourly contract, then the client can theoretically ask you to do all kinds of work, and that is fine as long as you log time for ALL of the work that you do. But the client can not force you to do anything. If the client asks you to do something new that had not been discussed before, then you decide whether or not you want to do it. If the client asks you to do something you don't want to do, then just tell him you don't do that:
"Jeremy, thank you for letting me continue to work on the project. I will be happy to draw a 5th cat. But you asked me to draw a mouse, and I don't draw mice. I only draw cats."
Managing your clients and your contracts are very important skills for a freelancer... As important as the actual work.
The way to respond to additional (beyond the clearly defined scope of the project which is your responsibility) requests is an enthisiastic "Yes, sure, great idea" followed by "I can get that done for you by Friday, the cost will be $ XXX."
Yahor, if this is a fixed price project, then did you actually tell the client - before you started working for them - how many concepts and revisions were included in your proposal? I used to bid on logo design projects on Elance all the time, and I saw many logo designers who promised things like, "Unlimited revisions!" and "I will work with you until you are completely satisfied."
If you do another revision, just say "this is the final revision." And if he asks for another close the contract and consider it a learning lesson. Re-read everything Petra and Christine said about managing clients. If you are going to be a freelancer, you need to learn to manage these client relationships and contracts.
To offer some additional perspective, to help you think about the possibilities, consider this:
I offer zero revisions.
With a fixed-price contract, I provide the client with a description of what the task will be. The client funds the task. I do the work, and submit it, without asking for the client to look it over before I click the green button.
I was hired because I how to do the task, and I know when the task is done.
I provide the client with everything necessary to test and verify the work. I provide the client with all files at all times.
The client releases payment when I say that the task is done. And then the client can fund the next task. Etc.
I am not saying that every freelancer needs to work the way that I do. But it is important to understand that a fixed-price contract is NOT an hourly contract. When a client just asks for whatever they want... that's an hourly contract. That is not how fixed-price contracts are meant to work.