First, note that the return value from a signal emission is the return value of the last handler executed. Since event signals are all of type RUN_LAST, this will be the default (GTK supplied) handler, unless you connect with the connect_after() method.
The way an event (say "button_press_event") is handled, is:
Emit the generic "event" signal. If that signal handler returns a value of TRUE, stop all processing.
Otherwise, emit a specific, "button_press_event" signal. If that returns TRUE, stop all processing.
Otherwise, go to the widget's parent, and repeat the above two steps.
Continue until some signal handler returns TRUE, or until the top-level widget is reached.
To prevent the default handler from being run, you need to connect with connect() and use emit_stop_by_name() - the return value only affects whether the signal is propagated, not the current emission.