There isn't a way to do that within the plugin itself, as it simply repeats the target action section fo the action sequence on the attack skill.
If you're using the YEP_X_ActSequence packs (which I highly recommend as they're free) you could wrap the entire action sequence in an if statement that checks if the target has more than 0 hp before performing the action sequence.
This isn't ideal, as you can't nest if checks in an action sequence without another plugin, so if your action sequence already has an if check in it, it won't work.
The default action sequence looks like this:
perform action wait: 10 action animation wait for animation action effect death break
So a dirty modification to your default attack skill would be to add the following notetag to the attack skill in your database:
<target action> if (target._hp > 0) perform action wait: 10 action animation wait for animation action effect death break end </target action>
I can't think of a way to stop the extra attack if the target dies otherwise. Battle Engine core makes it so that even if you deal 12000% of the enemies hp in one hit, the rest of the action sequence will continue to play out.