The Rabbis decreed that idols transmit tumah, ritual impurity. R’ Akiva refers to a source in Isaiah, but R’ Adin Steinsaltz explains that this is more of a memory aid than a proof text. The decree is a fence around the Torah prohibition to engage in idolatry of any sort because it is an abomination for a Jew. We don’t even rebuild a shared wall between our house and a house of idolatry so as not to benefit the practice of idolatry. Instead we move the wall inward from the boundary and create a buffer of weeds and bushes. The broken pieces of a smashed idol, however, do not transmit tumah because the Sages did not issue new rabbinic decrees to prevent people from violating existing rabbinic decrees.