Public spaces are created for the convenience of visitors, but frequently, their operation does not match individual visitors' needs. We propose instant social control as a new approach for controlling devices deployed in public spaces, envisioning the democratic and inclusive operation of the spaces. It is an approach to embrace individuals in the dynamic and unpredictable situations of public spaces, providing them with a powerful tool of immediate engagement and reflection. This research explores the use of instant social control, focusing on cases where discrepancies and conflicts arise among visitors' preferences for operating public devices. We conducted a field trial by deploying a technology probe to two real-world public spaces: a university auditorium, and a cafeteria. We collected usage logs and surveyed the users' experience with this new way of device control. We further investigated their experience through in-depth interviews with participants. Our field trial revealed a rich set of findings including voting strategies, exploratory device control patterns, users' attempts to communicate with other visitors, considerations of other visitors' discomfort, and acceptance and usefulness of instant social control of public devices.