CRM Service Activity - Issue with Javascript on save

I am attempting to stop the service activity form from submitting. I have created this small javascript method that is executed on form save. The last line of the method confirms that the save event has been prevented. However, the form still submits. Is it not possible to prevent form submission on service activities or am I doing something wrong?

function save(context) {
    var saveEvent = context.getEventArgs();
    saveEvent.preventDefault();
    console.log(saveEvent.isDefaultPrevented());
}