How can I disable the Design-Mode in VisualStudio options

In vs2015 goto:

Tools > Options > Xaml Designer

Choose "Source View" for "Default Document view"

or you can disable it completely by unchecking "Enable XAML Designer"

In vs2010 goto:

Tools > Options > Text Editor > XAML > Miscellaneous

Under "Default View" Section

check Always open documents in full XAML view

Right click .xml file in "Source Explorer", select "Open With" and choose "Source Code(Text) Editor" as default.