It might be worth avoiding this is you can by opting for Tobacco or Coffee plantions whenever possible to avoid swamping the market with too much sugar. However, it looks as though there is a minimum market price below which it will not fall, as it seems to have become steady at 15 per loaf.
A good tip. In my GB campaign I took all the Caribbean Islands away from Spain & France, greatly increasing my Sugar supply, and the price dropped from 16/loaf to 12/loaf. It would be nice to know which Plantations are optional for Sugar/Tobacco/Coffee so one can dismantle some of the Sugar Plantations and reduce the commodity glut.