A dioecious flowering plant prevents both:

When unisexual male and female flowers are present on different plants, the condition is called dioecious, and it prevents both autogamy and geitonogamy. Autogamy or self-fertilization refers to the fusion of two gametes that come from one individual. Xenogamy is the fertilization of a flower by pollen from a flower on a genetically different plant. Geitonogamy is considered if a first flower is pollinated by the pollen of the second flower of the same plant. Cleistogamy is self-fertilization that occurs within a permanently closed flower.