Most have already been said, aerobatic airplanes are designed to stall in a certain maner.

If you realy want to change the aeroplane behaviour there is at least two possibilities with a symmetrical wing section,

-Add a "trip" device at the wing root to stall this part of the wing prematurely.
(A flow distruptor as mentioned in first answer.)

-Add a wing "cuff"/ leading edge extensionto the outer part of the wing. No "wing droop" as the wing have a symmetrical section.
This is easy made out of foamwith a hot wire cutter. The wing section is unaltered but the wing area increased and the taper ratio decreased.