http://agathe.gr/democracy/ostracism.html
Ostracism In addition to the legal assassination condoned in the Law against Tyranny, a less extreme method was also available for removing powerful but dangerous men from public life. This was a formal, ... By these the citizens entered, each with a potsherd (ostrakon) on which he had scratched the name of the man who seemed to him most dangerous to the state. ... They were shoveled up and carried out to fill potholes in the roads leading out from the Agora. ... Another ostrakon, with the name of Kallixenos, who is not known to us from literary sources, designates him as a “traitor” (22).