I feel your pain...
I had the FBI virus, takes up the whole screen and totally locks up your computer till you pay a fee but as you said, no way of knowing if they will unlock your computer or come back with another request for more money :(
I found out that if I'm fast enough, had Windows 8.1, not have Win.10, you can click on the Windows button and shut down or restart your computer and they can't lock you up....
Thank you know who in the heavens for Windows 10 :)