Can anyone please tell me how to prevent users from deleting site elements, i.e. letters, comments, tasks, etc.?
I am using SharePoint 2016.
You would have to create a custom Permission Level that does not allow deleting. You would then grant all your users or groups this permission level. I wrote a blog post about that a while back, it is for SharePoint 2007, UI has changed, but the same principle still applies. To learn more check the official documentation on permission levels.