I have the problem that K2 articles still gets recognized with the wrong url form in my sitemap.xml. They are also get indexed by google with this wrong form.

I already have (hidden) menu entries which are pointing to the K2 categories, but still have this problem. Is there a possibility to disable the component/k2/articlename url form completeley?

Have you set Global Configuration / Site / SEO and K2 Settings, SEO?