You can see this in BasicLookAndFeel.java (or a similar class depending on the L & F you use), search on F6.
It looks like F6 is used by JSplitPane to switch focus between content and delimiters. To remove it, you can use something like (not verified, I think that deleting the actual action is harder because it might be in the parent input map):
splitPane.getInputMap(JComponent.WHEN_ANCESTOR_OF_FOCUSED_COMPONENT) .put(KeyStroke.getKeyStroke(KeyEvent.VK_F6, 0), "none");
source share