In order to forcefully disconnect a user from the database, you have to find the Process ID (pid) for the backend connection of that user. Each user connection to the database spawns a backend process for their connection. Once you know the pid for the process, you can kill the process. On unix the command would be "kill [pid]" while other OSes may have different ways to kill a process. In order to find the pid you can look at the pg_stat_activity table and the column procid should contain the number you need.