robots.txt is only for crawlers (search engine bots). Plus, it only works for those crawlers that respect the directives contained in robots.txt. It does not prevent anything, it just contains your preferences that you want crawlers to follow.
So, reputable search engines such as Google and Yahoo will respect and obey robots.txt. Many rogue crawlers will just ignore it.
And, if these bots are now search engine crawliers but are, instead, some sort of hack or control attempt, robots.txt is entirely irrelevant.
I am not WHB staff and I am not paid.
I provide help in these forums on my own time.