The incidence and risk of prostate cancer has been studied in many populations. These have included men who've had vasectomies and those who may or may not be sexually active. Some studies have suggested that men with a higher frequency of ejaculations may have a slightly lower risk of prostate cancer. However, this difference appears to be very small and is very difficult to accurately study.
At this time, there is no conclusive evidence that frequent ejaculation reduces the risk of prostate cancer.