Is there a way to turn off the "iCloud Drive isn't compatible with Mavericks" errors? I installed Yosemite on one of my machines (and uninstalled it a short time later). 

Now, running Mavericks, every time I open a program that uses iCloud (like Numbers), I'm greeted with this message. 



After clicking OK, I can still read documents stored on iCloud, I really can't see any functionality differences. After I pick a file, I get the same dialog box again, and click OK again, and I can edit and save my file.  

This is extremely obnoxious.

I would guess that I can't convert my account to non-iCloud Drive, which is fine, I don't really depend on "iCloud" for anything. 

My question is, how can I get rid of the error message?
Guesswork, but the bad news is you should have switched it off before reverting, if at all possible.  

Looking through the various QAs at Apple forums would tend to lead to the conclusion that once you have upgraded to iCloud Drive it cannot be reverted. You may be able to switch it off from https://icloud.com but I can't test as I have never enabled it on my machines.

Apple have never been too concerned about people wanting to rewind.

General iCloud Drive info from Apple doesn't cover it, nor does the switching off iCloud Drive FAQ