How to block unwanted calls and SMS on a Nokia Lumia 900? The phone language is set to German and I don't know how to do it.
Windows Phone 7, which is what the Lumia 900 uses, does not have the ability to block calls. Some carriers do have call blocking at the network level, so you may want to check. This is particularly true for abusive callers, as opposed to unwanted commercial callers. Here is a link from AT&T discussing this.
Windows Phone 8.1 includes call blocking, you can find out more in Settings.