There's pop siren Madonna, and then there's the Madonna--the Virgin Mary, that is. And Raffaele Santini--mayor of Pacentro, Italy, where the ancestors of the singing Madonna were born--declared Tuesday he knew the difference and thus promised there would be no semi-nude statue of the songstress erected in Pacentro's town square. The mayor's vow puts a crimp in the plans of sculptor Walter Pugni, who wants to create a 12-foot bikinied bronze statue of the singer and place it in Pacentro as "a symbol for our children . . . of a better world for them in the year 2000." Pacentro's five churches are dedicated to the other Madonna. Santini replied that the townspeople adequately paid tribute to Madonna when they made her an honorary citizen during her visit to Italy last September for a concert in Turin.