The Shulhan Aruch (OC 179) rules that if one already washed Mayim Aharonim and then decides he wants to eat more, he may not do so, even if he makes a new Beracha. Rather, he must continue to recite Birkat Hamazon. However, in a case where he did not yet wash Mayim Acharonim, but merely announced "Hav Lan Nivrach"-Let us recite Birkat Hamazon, he may continue eating provided that he makes a Beracha on the new food. That formal announcement constitutes a "Hesech Hada’at"-an interruption of the meal and severs the connection to the original Hamosi.
It is important to note that only the Ba’al Habayit-the head of the house can terminate the meal by his statements. A guest cannot control when the meal ends. Therefore, even if a guest made a declaration to end the meal, he is still dependent on the Ba’al Habayit and may continue to eat without a Beracha.
Nevertheless, nowadays when the head of the house tell his family to bring the Birkonim or the Mayim Acharonim, it is not an official end to the meal. He is merely giving his family a "heads up" that the meal is concluding. Therefore, he may continue eating again without a new Beracha. Rav Elyashiv (1910-2012, Jerusalem) is quoted as explaining this Halacha that nowadays, it is more common for people to change their minds about ending a meal, and a statement of this sort is not necessarily a person’s last word. Only the act of washing Mayim Acharonim or the actual Zimun terminates the meal.
The best practice is to avoid such a situation in the first place, since the Ben Ish Hai (Rav Yosef Haim of Baghdad, 1833-1909), in Parashat Korach, does say that requesting to bring the Mayim Acharonim constitutes a termination of the meal.
SUMMARY:
Once a person washed Mayim Acharonim, he may not continue eating, even if he were to make new Berachot.
Nowadays, a statement by the Ba’al Habayit that he intends to end the meal does not terminate the meal and all present may continue to eat, without making new Berachot.
