Add .dropboxignore directory to exclude folders without using selective sync

I couldnt find any other question like this.  I want to exclude some folders from dropbox.  I know that selective sync allows you to not sync certain devices with dropbox folders, but that doesn't help me because that doesnt affect dropbox itself.  

Selective sync is good if you have a device that does not have a lot of space and you don't want to sync your Camera Uploads to that device to save space, for example.

But my problem is I don't have much space in dropbox because a subfolder (Folder B) of one of my dropbox synced folders (Folder A)  is massive.  Folder A is a very important supra folder that I want to sync, but if I could exclude Folder B, it would mean the rest of the valuable files in Folder A and its other subfolders would be safe, but I would regain that space that Folder B was unecessarily hogging. 

Is there a way to do this?

Thank you!