PHNOM PENH Cambodia's influential former leader Hun Sen yesterday issued an ultimatum to Thailand to lift border crossing restrictions or his country will ban all Thai fruit and vegetable imports in the latest round of the border spat between the two countries. One Cambodian soldier was killed on May 28 as troops exchanged fire in a disputed area known as the Emerald Triangle, where the borders of Cambodia, Thailand and Laos meet. The Thai and Cambodian armies both said they acted in self-