In a food processor, puree mango, adding 1 tbsp water, if necessary, to achieve a puddinglike consistency. Add spices; blend thoroughly. Add sugar to taste. Cover and chill 3 to 4 hours. When ready to serve, cut watermelon in half. Using a melon baller, scoop out about 2 cups of balls. Pour puree into 4 shallow bowls and arrange melon balls on top. Garnish with mint sprigs.