Can I prevent users from deleting emails and documents?

The delete function is available for all standard users in each document list.

But this is only a "soft" delete. The right to permanently delete is only available to FYI Admins from within the "Deleted Documents" view. Standard users can use this view to locate documents they have deleted and restore if necessary.

Users cannot delete (or edit) emails and documents when they are in Co-edit, being edited by another user or have certain workflow states. Refer to the FAQ Why can I not Edit or Delete an email or document?

Refer also to Deleting Documents and Recovering Deleted Documents and Deleting Emails and Recovering Deleted Emails.