Hi Christian Bischof,
Actually, that's what I asked myself from the first time I saw your question: why would you want to have a marker that is partly clickable, and partly non-clickable at all?!
From an end user perspective: it would drive me nuts if I clicked on a marker (because visually it is just one marker) and it would only react when I clicked in a particular part of it.
That's why click events which happen "somewhere very close to it" are also honored. It would discourage the users of your app if they have to click 10 or 20 times to open a popup...
So, anyways, you could try to use the suggestion which I gave above, but most probably it is best not to use it at all.
BR,
Egge-Jan