I know that some devices (newer Sonys) will remove the title bar if you set the Displayable title to null.
I have no idea if this has any effect on Series 60, or exactly what it would change.
This is about as much control as you get in J2ME. Remember that the API is designed to function on any platform, so you don't get fine control over platform-specific aspects.
You can take complete control, by using a Canvas with setFullScreenMode(true). Then you can paint whatever user interface you want. But: you have to do all the work.