Three people have been banned from visiting prisons after being caught apparently trying to smuggle in cannabis to inmates.
Kent Police and prison staff carried out an operation on the Isle of Sheppey, which is home to HMP Elmley, HMP Swaleside and HMP Standford Hill.
Officers used automatic number plate recognition (ANPR) to check more than 750 vehicles of prison visitors.
Insp Alan Rogers said illegal activity in prisons would not be tolerated.
A Kent Police spokeswoman said two vehicles were seized for having no insurance and several fixed penalty notices were issued for other offences.
Three people were given police warnings for possessing cannabis and banned from visiting prisons, she added.