Our company has 5 servers ranging from Domain Controller, File Server, Application server, Database server and Email server. The problem is when a staff resign or unhappy with the management they delete some of the files in one of the servers.

So how do we prevent users from deleting files from the server. We just want the users to be able to save files but not deleting them.

Any advice is very much appreciated.

You can't. If a user has the rights to save (modify) a file then by saving gibberish or blanks the file is effectively, if not physically deleted. All you can do is ensure that backups are done frequently and have employees (or ex-employees) punished severely and publicly.

it's quite hard, because if you allow them to write only. Then the cannot modify their files.
Do a regular backup, or enable shadow copies..

Buy a cheap NAS, just for backup storage of those important files and set a task scheduler do a robocopy to the NAS or make use of wbadmin command line tool (free alternative solution)

Here's a radical thought. Why not have a talk with the employees to find out what management is doing to piss them off to the point where they would consider breaking the law to get even? If this is a frequent occurrence then either your management is over-burdened with a$$holes or your company is not doing a very good job screening their employees, or both.

I rather deal with machine than human. Humans are more complicated but only if I could just do that than my life is easier. Thanks to all who replied.