Mating plugs have been described in many species, and their presence often implies a function in protecting a male's ejaculate. Yet, explicit functions are not always tested. In this study, we test whether fragments of male genitalia lodged in the female genital opening of the St Andrew's Cross spider (Argiope keyserlingi) are mating plugs and prevent female remating. Further, we test whether copulation duration, cannibalism, and male or female size affect the lodgement and persistence of these genital fragments. We show that males always break off a genital fragment, which when lodged in the female genital opening, can successfully prevent female remating. However, the lodgement of a genital fragment is not always successful and it may not persist for a prolonged period. Whether a genital fragment is successfully retained is influenced by female control over copulation duration. We have previously shown that females can terminate copulation duration by attacking the male, which may or may not lead to cannibalism. If females terminate copulations early, genital fragments are either not lodged or do not persist. Male size can offset female control with larger males lodging more persistent fragments. Contrary to predictions, sexual cannibalism was not related to how long the fragment persisted within the female. We demonstrate the existence of mating plugs in St Andrew's Cross spiders and document considerable variation in the formation and persistence of mating plugs that is likely to reflect male and female conflict over mate plugging.