German keyboard Alt-D gives '∂' - how can I disable it? It should be an easy thing to do if I only could find out how to change the symbol (e.g. from ∂ to € or so) - but as a programmer with a lot of shortcuts this is the only thing that keeps doubting me, if a Mac is the right choice.

It's a 10.10.3; I'm using Intellij and Emacs.

Btw: the same happens for lots of other keys (€®†Ω¨⁄øπ•œ@∆ºª) - this makes the alt-key almost useless.
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].