Code:
on 1:join:[color:red]#channel[/color]: { set %fkey.join $nick }
alias F[color:green]5[/color] {
  ban -k $chan %fkey.join 2 Banned
}


In red is where you set your channel. in green is the F key you want, this will only work on the last joined user, I also dont recommend using this on large channels where a lot of people join and part. This code goes in the remote editor (ALT+R)