Limitation collar

A limitation collar was a form of restraint placed on visitors to the Leisure Hive on Argolis who were accused of criminal activity.

The collars were programmed to allow the wearer to roam freely around the Leisure Hive, but forbade them access to certain areas. If the wearer entered a forbidden area, the collar would tighten around their neck, strangling them, and would also contract if the wearer attempted to remove it.

The Fourth Doctor and Romana II had limitation collars placed on them by Vargos by order of Mena, after the Doctor was accused of murdering Stimson. They were freed by Hardin using a key he had borrowed from the Leisure Hive's Security Office. (TV: The Leisure Hive)

Behind the scenes
The areas of the Leisure Hive to which the Doctor and Romana are forbidden access while wearing their limitation collars are never mentioned by Vargos on-screen, but in David Fisher's novelisation the Argolin tour guide details these as being the Great Recreation Hall, the laboratories, the Board Room and the Shuttle Station.