file security

NTFS Advanced file permissions.

Go into the Security tab, click Advanced. Select the users or groups you want, and click Edit. Uncheck the Delete special permission, while making sure they still have Modify. You can use the deny setting, but remember that deny overrides everything else so if you deny Delete to Authenticated Users, nothing will be able to delete the item.

Also, make sure that the users in question don't have Change Permissions or Take Ownership, since that allows them to gain the Delete permission.

Note that the Modify permission allows users to make a file useless or indirectly "delete" it, such as deleting the contents of a Word document and saving it.