Activity Monitor
Tools > Activity Monitor 

A few different tools within SGA allow you to see what users are logged into the system and end their processes.

The activity monitor is a tool that shows which users are currently accessing the SQL server and how many processes they are running. It summarizes the processes for the SGA user, NT user, and Login or Windows user. A process is not how many times a user is logged in but how many jobs the database is running for that user, which is usually several.

Right-click a user to see all the processes for that user, or click the Databases button to see a list of all databases or kill all processes for a database.

Processes

This shows processes, which is a database's most detailed level of activity. There are also filters at the top of the screen to narrow down your results.

Databases

This shows a list of all SQL databases on the server and whether they are online or offline. Right-click and select Take offline to take a database offline so that no one can access it for a period of time. Right-click and select Kill all processes for database to kick any user accessing that database out.