Maybe its a silly question but can Telegram just give back $1B to all american participants of the project and continue the process?
That’s called disgorgement of proceeds. There are also usually fines and agreements not to break the rules again.
In this case, however, it would be tough to ensure no future buyer of these securities is American. (To say nothing of Telegram having raised U.S. dollars and so falling under U.S. jurisdiction.)
They already spent a good chunk of it: by Jan 31 2019 they spent $218M (that's a LOT! 200 employees burn no more than $50M a year, hosting maybe another $20M; the rest is probably just taken out by insiders). At that burn rate they likely have spent half a billion by now. Who is going to cover that?