Sync issue with Basic plan after backing up an external drive

My basic account has no more than 167MB files in Dropbox storage.
But sync doesn't work referring to the fact that I have already used all available space under my plan. It happened some days ago when I attached  an external USD disk-storage to my home computer and DropBox automatically started downloading of all files on it to Dropbox storage, though I didn't ask him about that.
I cancelled default option of syncronizing external disks, but the Dropbox still refuses to sync files between my devices, as it did before. 'Cause still - lack of space. I guess the biggest part apparently is currently in use by the files that were already syncronized by the moment I cancelled it.
Being in my shoes, what would you do to fix that?
I can't even find a storage where files from my external disk were downloaded to delete them and free some space in the Dropbox cloud for the files I want to be sync'd between my devices.