Oracle 11g introduced the ALTER SYSTEM DISCONNECT SESSION syntax as a new method for killing Oracle sessions. Unlike the KILL SESSION command which asks the session to kill itself, the DISCONNECT SESSION command kills the dedicated server process (or virtual circuit when using Shared Sever), which is equivalent to killing the server process from the operating system. The basic syntax is similar to the KILL SESSION command with the addition of the POST_TRANSACTION clause. The SID and SERIAL# values of the relevant session can be substituted into one of the following statements.
If after a few minutes the process hasn't stopped, terminate the session using the following.
kill -9 spid