Ronny,

If you apply the style to the following test:

Hello World

Hello World 

In the HTML view, you will the editor achieve that by using the span tag:

<SPAN class=Heading>Hello World</SPAN>

Romove format button can't remove this class attribute. If you click use the Remove MS Word formatting button, it will remove the class attribute. Then you can apply another style. The only problem is Remove MS Word formatting button will not remove the span tag.

I hope it helps. If not, please post here.