As stated before, the laws of the hosting city are in affect on the premises of the fairgrounds and should be strictly enforced.
A majority of the crimes seen during the fair is theft and disruptions.
In the event of a disruption, first ask the aisling to stop and give warning of the punishment that will be enforced if they do not. If the disruption continues, a teacher may be able to remove them from the area temporarily. If not, they should be banished or exiled for the duration of the fair at least. This will prevent any further disruption from that aisling.
For theft, if possible have a rogue that has the ability to "peek" take a look inside the pockets of the alleged thief to determine if they do have the item. Try to talk them into giving the item back. If they refused, the should be punished immediately under the hosting city's laws.
Peasants are a bit of a different story. Many aislings hire peasants to break the laws and cause disruptions during the fair. Currently, peasants should not be able to purchase seals of any kinds though this should be tested. This should reduce the thefts from "hiding" peasants. Peasants with offending names should be banished on site as they are just there to create a disturbance.
Any official that knowingly issues seals to a criminal should also be looked at as a criminal and immediately removed from office by other officials. The committee planning the fair should make those issuing seals aware of the aislings that may purchase director's seals. Director's seals should not be sold to anyone without prior permission of the planning committee.
The current law for the fairgrounds is as follows and
may be used as an example for any additional laws.