You could have done it like so:

on $@*:text:/(^| )(http\072\/\/w{3}\.|w{3}\.)(?!hawkee|google|youtube).+(\.\S{2,6})( |$)/Si:#:{
  kick # $nick The URL you posted is not allowed in here!
}

This will exclude URL consists of hawkee, google and youtube. The domain extension can be as long as 6 characters, not just 3.