How could one calm down an over-active sympathetic nervous system?

See below. First, not clear what you mean by over-active, as to me it could imply reflex sympathetic dystrophy, hyperthyroidism, even hypertension. Second, you need to explore possible underlying disorders which might cause disruption of autonomic nervous system. Third, is your lifestyle or food/alcohol/smoking an issue. Talk to your doctor and explore your concerns.