johnfm, one simplest way you can do is kill via the snotice event when they connect to your server:

on $*:snotice:/\d\:\s(\S+)/:{
  var %bnick nick1 nick2 nick3 nick4 nick5 nick6
  if $istok(%bnick,$regml(1),32) { kill $regml(1) Bad nick detected! }
}

Substitute nick1, nick2, etc...with the relative ones you want to look out for.