There seems to be a way now that's in beta, worked on windows 10 although doesn't show up as working until I pause sync and resume.
You have to navigate to folder on dropbox, hold ctrl+shift+(right click) somewhere in folder, [open powershell window here]
Then inside you copy/paste:
Set-Content -Path $Pwd -Stream com.dropbox.ignored -Value 1
So basically $Pwd is path to this location in powershell and the rest flags it somehow to ignore
This will (on next sync) put a grey minus sign on the whole folder and seems stop syncing. I've lost so many files to dropbox sync that happy at least something works. Found the solution elsewhere for files (replace $Pwd with path that includes filename at end) but I wanted a reusable script I can do quick.
Hope this helps.