Here is a basic script to do what you asked:
on @5:TEXT:!opme:[color:green]#channel[/color]:{
mode # +o $nick
}
on 10:TEXT:!addaccess & &:?:{
guser $2 $3 2
msg $nick Added $3 to access list with level $2 $+ .
}
on 10:TEXT:!remaccess &:?:{
ruser $2 2
msg $nick Removed $2 from access list.
}
!addaccess 5 bob would allow bob to use the !opme command.
!addaccess 10 bob would allow bob to use the !opme command, !addaccess and !remaccess commands.
!remaccess bob would remove bob's access to all commands.
As for your other question the answer is no, ischan is only used to see if you are on a channel, not anyone else. If you want to ban all users who join your channel who are also on an "enemy" channel try Racoon's script
here.