Theres a section called "disallowed commands" in languages/chat.php (~line 23), make it read:
This will make it so users cannot use the "nick" command to change their nicknames.
Then you can also remove the "nickname" box by editing templates/default/chat.tpl by changing this line (~line 20):
<param name="pixx:nickfield" value="true">
to:
<param name="pixx:nickfield" value="false">
There are a LOT of integers in the chat.tpl file, as stated in the main post I made just about every option there is with the pJIRC script available there for "advanced" users.
A quick FYI, you can use the command filter to filter the join command as well to restrict people to one channel; an alternative for this would be to use the channel join filter (in the chat language file, ~line 21) to filter "none+#yourchannel" or "none+#yourchannel+#yourchannel2".