How do I get rid of killed sessions (remaining in status KILLED)?

Our Database: Oracle Database 11g Enterprise Edition Release 11.2.0.4.0 - 64bit Production

I am trying to kill a session using the alter system kill session 'sid,serial#' immediate -command.

Result: Session is killed (status = 'KILLED') but does not disappear from v$session for a very long time (until restarting the database).

The spid from v$process doesn't exist anymore on our 64-bit Linux-Server ("ps -ef|grep <spid> |grep -v grep" doesn't find a corresponding process)

The serial# number of that killed session is growing continously (so any try to kill the session again is resulting in ORA-00030, because the serial# doesn't match the given one). Nevertheless the session is present in v$session (with a growing serial#).