My players seem to have found an exploit where a mob spawner was in a worldguard protected area, they kept breaking it, receiving money and it would just pop back up and they would do it all over again. At any rate, I think to fix this problem you would simply need to change when you register the Block_Break event from NORMAL priority to MONITOR. That way by the time your plugin sees it, all changes that other plugins such as worldguard are already made, therefore if worldguard cancels the event, ecoCreature does not give a reward.

Thanks for the report. Changed it to Monitor.