Disabling all but one child control in a WPF window

You can put all the controls in one panel (Grid, StackPanel, etc.), and leave the cancel button in another panel. Then set the IsEnabled property of the other panel.

In practice, this will probably introduce more than one additional panel.

For example, if you had a StackPanel of buttons, you can add an additional StackPanel:

<StackPanel Orientation="Horizontal">
    <StackPanel x:Name="controlContainer" Orientation="Horizontal">
        <!-- Other Buttons Here -->
    </StackPanel>
    <Button Content="Cancel" />
</StackPanel>

Then, you would do the following to disable everything but the cancel button:

controlContainer.IsEnabled = false;

I also wanted the user to be able to cancel out of loading. I found a lovely solution.

foreach (Control ctrl in this.Controls)
    ctrl.Enabled = false;

CancelButton.Enabled = true;

This also allows the main window to be selected and moved unlike this.Enabled = false; which completely locks up the window.