Irule to block request from amazonaws.com

The issue here is that amazonaws.com is not the Host value. The amazonaws.com bot is making a request to your site, so the Host value is still your HTTP Host. To find a crawler bot, you'd want to use the User-Agent header.

when HTTP_REQUEST  {
    if { [matchclass [string tolower [HTTP::header User-Agent]] contains blacklist_host] } {
        reject
    }
}

But then it may also be useful to consider using a robots.txt file: https://developers.google.com/search/docs/crawling-indexing/robots/intro, which you could host directly from an iRule:

when HTTP_REQUEST {
  if { [HTTP::uri] == "/robots.txt" } {
    HTTP::respond 200 content [ifile get robots.txt]
  }
}