Regular ERS deployment is as some said, done automatically. It is GPS based. However, there have been many times in WEC where drivers have used an extra boost to overtake. If this was outlawed through new regulations then I missed it.
I would imagine that drivers can override the deployment but only if they really must, since being limited to a certain energy usage per lap, they will lose that elsewhere. Also, teams will have set the system to deliver best possible lap times.
So to sum up... I'm not totally sure.