When closing Lightroom, I inadvertently approved backing up photos to dropbox. Now my dropbox is full and I cannot access the files that are uploading nor can I delete them.

I can't use my dropbox now that it is full. When I create space by deleting other files, the uploading "Master.apmaster" files fill the dropbox up again.
I'm frustrated that I approved this process but I'm equally frustrated that this option was offered to me.