Nextcloud Client, dont syncronize deleted Files from Client in Cloud

We have set up nextcloud on a vServer for our class. All of them synchronize the cloud to your PC using Nextcloud’s client tool.

We have a group folder that all syncronize. Now, of course, the problem is if someone on his client deletes a file, the delete is also passed up to the cloud ,and deleted in the cloud. Is there a way to control that the cloud is the master and can only delete a certain group of files?