I had this problem and putting a file in the folder did not resolve it, it immediately re-created the folder after I deleted it from the Finder (Mac 10.13.3). What did work was to choose Pause Syncing from the Dropbox menu bar widget, delete the file in Finder, then create a new folder with the same name (this is what I was trying to do in the first place). After resuming syncing, it left it alone.