Prevent Unknown robot (identified by 'bot' followed by a space or one of the following characters _+:,.;/\-)

I have a drupal operated small website. It is not commercial, ie: no income from it. The site is in shared host, so has limited bandwidth and resource. This site encounters "Bandwidth exceed" problem every month because more than 60% of bandwidth is being used by bots and spiders. The maximum is gone to:

Unknown robot (identified by 'bot' followed by a space or one of the following characters _+:,.;/-)

How could I identify or block the bad bots? Please suggest me some way to prevent it.