I list some common places to look --
- System -> Preferences -> Network proxy.
- Open terminal.
$ echo $HTTP_PROXY
If it still shows the old value reset it by the command
$ set HTTP_PROXY=""
$ export .bashrc
Do the similar thing for other shells csh etc.
- The settings for KDE ~/.kde/share/config/kioslaverc
$ cat ~/.kde/share/config/kioslaverc
If it still shows the old value in httpProxy then edit it. Do not forget to change the value of PersistentProxyConnection.