Making the contents of a folder undeletable is easy, you can add an access control entry (ACE) on the Everyone group denying delete. Deny overrides Allow, and all users belong to Everyone, so now no user can delete. Admins are not excepted from this, so they would not be able to delete.

If you need admins to be able to delete, then what you really want is for most users to not inherit rights to delete the contents of the folders. At some point in your folder tree, you must have granted users the right to delete in a way that inherits down to the folder you want to secure. Can you change that ACE, or move the folder so that it is no longer under that ACE? It may take some planning, but the best way to manage security is to try to not grant more rights than you need to, because selectively removing them is tricky.