This is known "limitation" of JBoss Portal. If you deploy war containing *-object.xml, it's a one way process unless you use Portal's admin UI. Simply undeploying the war wont get rid of the tab.
Thanks for that information - it seems that if I go to the Portal Object tab in the admin, then the Default object, and delete it from there it is no longer on the front page.
I could swear I looked there yesterday but this morning's fresh eyes saw it!
Thanks for the clarification too :)