A boundary stone
The Agora was formally defined with a series of boundary stones around 500 BC, placed wherever a street entered the open square. These two are still in situ, and are inscribed "I am the boundary of the Agora". (Note that the Greek is written "backwards" as was normal at this early period.)
As well as women and males under 18, certain types of offender were banned from the open space: cowards, traitors, those who mistreat their parents, and "those who do not have clean hands" (Demosthenes). Basins for purification at the entrances emphasised that the Agora was a religious centre as well as a commercial one.