Share Folders or Network Shares Only Online - No Sync with Desktop

Is there anyway to share a folder, like a group folder, or a network share, that can ONLY be accessed online? I do not want any of my clients ever to try to sync it with the desktop (Win, or Mac). I tried this earlier and it worked for a while, but then my daughter accidentally turned on sync and it tried to copy my family Shared Drive on a local server on to her tiny PC. Filled the HD immediately. Any ideas? I love the sync with personal drives, but I have a NAS running with 20 TBs of photos from all our phones over the years. I would love to access through Nextcloud, but only ONLINE, never a local sync? Is that possible? Thanks for any tips.