How to prevent users from Deleting FTP files but only uploading / downloading?
I have a scenario where I allowed my users to connect to my FTP server with a sub-FTP folder account I created with user & password, and now every users are accessing the same sub folder and using the same user & password to upload / download data for sharing.
How do I restrict the users to not delete any files by mistake / accident / abusive act?
FTP by its nature is open to abuse and mistakes. FTP is really meant to just transfer files, not to really fully "manage" user access. FTP is a very primitive, but vital part of the internet. Without knowing anything about how you've constructed your website, on a broad level I would recommend installing some sort of "File Manager" for your site. For example, in WP you could use something like EZDigital Downloads to provide downloads and Gravity Forms or Contact Form 7 to allow for uploads.
In the event, that you stick with FTP, you'll need to educate your self on the read/write/execute permissions at both the folder and file level for your site. You'll also need an FTP client that has the ability to modify a file/folder for permissions. Each time you add a file or a folder, you will need to appropriately set the permissions.