German keyboard Alt-D gives '∂' - how can I disable it?
The Alt key on the Mac is used differently than on Windows. Its main use is exactly what you see: To enter special ("alternate") characters.

Menu shortcuts and such are always triggered by a Command-key ("⌘") combo (sometimes including other modifier keys, such as the Alt key). For example, closing all open windows in Finder is [Cmd]-[Alt]-[W].