Allow web-access only (no sync clients)

[devnull]

I think it is not a good idea and i also think it does not work good. Nextcloud uses WebDAV and the user can use different client-software (file explorer, …)

Read:

How to disable desktop sync for certain Groups/Users/Folders?

Agreed, there are also times when you want to allow contribution but prevent theft. For example, remote employees contributing to work product that is either confidential or intellectual property of the employer. Blocking sync/downloads is important to prevent them from keeping “backup” copies after termination.