Get rid of the XAML window from the taskbar

If I have a XAML window that I am showing using ShowDialog, is there a way to make it (not) appear in the taskbar?

The problem is that I can set the window to be the topmost one, but if that window opens another dialog and I set it to be the topmost one, I can place the second one after the first by clicking on the taskbar. This tends to confuse users.