the top bar management is always under the operating system control. You can use full screen mode to cover full screen, however some diveces can still show the bar. Another option is to use Game Canvas instead of Canvas which might on some devices utilize even "more full" screen mode.
Some manufacturers add special .jad properties to utilize really full screen.
Summary: this is really manufacturer/device dependent.
Of course you can build your own UI completely - simply using the Canvas and paint it on your own. There are several UIs available for Java ME. However the most comprehensive and free is probably the LWUIT supported directly by Oracle. See: [http://lwuit.java.net/|http://lwuit.java.net/] and/or [http://www.oracle.com/technetwork/java/javame/javamobile/download/lwuit/index.html|http://www.oracle.com/technetwork/java/javame/javamobile/download/lwuit/index.html]