Creating Theory exercises

You can turn them invisible, either by using the checkbox in the inspector or by using the shortcut V

But also, depending on what you are trying to do here, there are probably better ways than creating a 12/1 time signature. Unless that really is musically what you want to show, you could instead use the "real" time signature and then use Tools / Measures / Join to combine multiple measures into one. Thus creating the same visible effect but without the 12/1.