By Julia Payne LONDON (Reuters) -Global commodities trader Trafigura Group will stop all purchases of crude oil from Russia’s state company Rosneft by May 15 when tighter EU rules on Russian oil sales come into effect, a company spokesperson said on Tuesday. The Geneva-based firm will also “substantially reduce” the volume of refined products it buys from Rosneft. Trafigura , along with rival Vitol, is a major lifter of Russian oil, mainly from Rosneft. The company and other major trading firms previously said they would only fulfil existing contracts and not agree any new deals. Trafigura and…