As so often, the solution is called "Backup".
Also you could not store your documents in the "My Documents" folder, make a folder on your C drive, store your docs, pics & important stuff in that. So if you do get cryptoransomed they will have done the wrong files.
That will only take you so far. With so many programs defaulting to the My Documents folder, it'd be annoying at best to have to point to c:\realdocs "because viruses". The user could point the "My Documents" folder to c:\realdocs, but now we're in the same boat again. Even if a user decided it was worth the hassle to deprecate the use of the system variable, c:\realdocs would still be accessible by the same user. From Windows' security standpoint, there's no difference between the user being attacked by ransomware, and the user adding a password to an Excel sheet. Thus, ransomware doesn't need root privileges to mess up a user's files.
Even beyond that, the next generation of ransomware wouldn't exactly need a foundational rewrite to go to %user%\recent and see where those files point to and encrypt all the .docx, .xlsx, and .qif files there. I'm sure that somewhere in userland, there's some indication as to where the Dropbox/OneDrive/Gdrive folders are, and encrypting all that stuff. Even less complicated would be to search all available hard drives for user generated file types. .dll files wouldn't be worth it, but .qbw files very much would be. Ultimately, trying to thwart an attack of this nature would be of limited success, because from the most literal of standpoints, the virus is doing nothing different than what a user would be doing.
Amongst the things that makes this kind of attack so successful is that very problem: if you're trying to prevent outbound traffic at the firewall, you've already lost, basically. How does security software distinguish. technically, between a cryptovirus taking a file hostage, and a user passwording a file with WinRAR and uploading it to SpiderOak? That, good friends, is a question that I pay ESET a nontrivial sum to discuss and determine.