Let's say on a short winter Shabbos you had a large meal on Friday night and another large midday meal on Shabbos day. The midday meal lasts a few hours and now it is close to the end of the day. You know it is an obligation to eat 3 meals on Shabbos, but the thought of eating anything else now is repulsive to you. Can you say a bracha over bread when eating it would be repulsive to you? What do you do?
If eating is actually repulsive, at that point it is not considered halkhicaly eating and he will not fulfil his obligation of eating the third meal. See Mishna Berurah siman 291 #4.
As far as making a bracha on just such a non-eating, see Mishna Berurah siman 204 #48 concerning a food which one finds repulsive but is eating for health reasons, being that he gets no enjoyment from eating it, one does not make a blessing.
If however the person is not at the point of repulsion, but rather just not hungry, although he is no longer commanded to eat that third meal because the mitzvah of eating the meals was given to be enjoyed, not to be bothersome, see Mishna Berurah in 291 #3, apparently he may still eat and make blessings.
Shulchan Arukh OC 291:1
יהא זהיר מאד לקיים סעודה שלישית ואף אם הוא שבע יכול לקיים אותה בכביצה ואם אי אפשר לו כלל לאכול אינו חייב לצער את עצמו והחכם עיניו בראשו שלא ימלא בטנו בסעודת הבוקר כדי ליתן מקום לסעודה שלישית:
One should be careful to fulfill The Third Meal, and even if he is satiated he can fulfill it with an egg's bulk. And if he is unable to eat at all he is not obligated to pain himself. And [a wise person thinks ahead] not filling his stomach in the morning meal so that he has room for The Third Meal.
Note the midday meal is The Third Meal. People would usually eat some food in the morning and at night. The special part of Shabbat was not working when you could eat lunch. Yum!