The key combination Ctrl+C replaces the selected text with a special symbol. This is reproducible only if the Ctrl+C opens the Silverlight dialog that asks you if you want to allow cliboard operations. Also, the Mask should be set to an empty string.

Workaround:
Subscribe for the ValueChanging event of the masked input control and if the new value equals one of the special characters from the ASCII table (you can get the character using the following syntax '(char)3' ) then handle the event.

void maskedInput_ValueChanging(object sender, RadMaskedInputValueChangingEventArgs e)
       {
           if (e.NewValue != null && ((string)e.NewValue).Length == 1 && ((string)e.NewValue)[0] == (char)3)
           {
               e.Handled = true;
           }
       }