No version of Oracle Forms allows you change these properties of a tabbed canvas. If you were using Forms 10g or higher, you would be able to use the Oracle Forms Look and Feel Project which is a set of JavaBeans created by Francois Degrelle and enables you the change many of the properties that Forms won't let you change. Unfortunately, you are stuck with what is available in Forms 6i.
Pls Try the following in WHEN-TAB-PAGE-CHANGED Trigger (Not tested..)
tb_pg_id := FIND_TAB_PAGE('tab_page_1');
IF GET_TAB_PAGE_PROPERTY(tb_pg_id, enabled) = 'FALSE' THEN
SET_TAB_PAGE_PROPERTY(tb_pg_id, enabled, property_true);