To modify the access permissions of a read only directory in order to delete some or all its files will be possible in Mustang (Java 6.0, see the [url http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=6216563]Bug ID #6216563[/url], also see the [url http://download.java.net/jdk6/docs/api/java/io/File.html]File new setXxx methods[/url]) and even improved in Dolpin (Java 7.0, see [url http://jcp.org/en/jsr/detail?id=203]JSR 203[/url]).
In the mean time you could do an OS specific call (e.g. chmod) with Runtime.exec().
Then of course to delete a file or (empty) directory you simply use [url http://java.sun.com/j2se/1.5.0/docs/api/java/io/File.html#delete()]File.delete()[/url].
Regards