How to purge data in group libraries
                    I reached the 6GB storage limit. I locally removed all pdfs above some size, so my local storage is now 5.6 GB. I would like to erase the server storage by this reduced local version. However when I use the "Replace Online Library" command it starts syncing and stops immediately saying my quota is overpassed. What to do ?                
                            
 Upgrade Storage
If you're saying you deleted files on disk, that has no effect on your Zotero library, other than making the files unavailable locally.
Do not use "Replace Online Library" — it's not at all appropriate here.