Put this into robots.txt in the root of your site:

User-agent: Rogerbot 
User-agent: Exabot 
User-agent: MJ12bot 
User-agent: Dotbot 
User-agent: Gigabot 
User-agent: AhrefsBot 
User-agent: BlackWidow 
User-agent: Bot [EMAIL="craftbot@yahoo.com"]mailto:craftbot@yahoo.com[/EMAIL] 
User-agent: ChinaClaw 
User-agent: Custo 
User-agent: DISCo 
User-agent: Download Demon 
User-agent: eCatch 
User-agent: EirGrabber 
User-agent: EmailSiphon 
User-agent: EmailWolf 
User-agent: Express WebPictures 
User-agent: ExtractorPro 
User-agent: EyeNetIE 
User-agent: FlashGet 
User-agent: GetRight 
User-agent: GetWeb! 
User-agent: Go!Zilla 
User-agent: Go-Ahead-Got-It 
User-agent: GrabNet 
User-agent: Grafula 
User-agent: HMView 
User-agent: HTTrack 
User-agent: Image Stripper 
User-agent: Image Sucker 
User-agent: Indy Library
User-agent: InterGET 
User-agent: Internet Ninja 
User-agent: JetCar 
User-agent: JOC Web Spider 
User-agent: larbin 
User-agent: LeechFTP 
User-agent: Mass Downloader 
User-agent: MIDown tool 
User-agent: Mister PiX 
User-agent: Navroad 
User-agent: NearSite 
User-agent: NetAnts 
User-agent: NetSpider 
User-agent: Net Vampire 
User-agent: NetZIP 
User-agent: Octopus 
User-agent: Offline Explorer 
User-agent: Offline Navigator 
User-agent: PageGrabber 
User-agent: Papa Foto 
User-agent: pavuk 
User-agent: pcBrowser 
User-agent: RealDownload 
User-agent: ReGet 
User-agent: SiteSnagger 
User-agent: SmartDownload 
User-agent: SuperBot 
User-agent: SuperHTTP 
User-agent: Surfbot 
User-agent: tAkeOut 
User-agent: Teleport Pro 
User-agent: VoidEYE 
User-agent: Web Image Collector 
User-agent: Web Sucker 
User-agent: WebAuto 
User-agent: WebCopier 
User-agent: WebFetch 
User-agent: WebGo IS 
User-agent: WebLeacher 
User-agent: WebReaper 
User-agent: WebSauger 
User-agent: Website eXtractor 
User-agent: Website Quester 
User-agent: WebStripper 
User-agent: WebWhacker 
User-agent: WebZIP 
User-agent: Wget 
User-agent: Widow 
User-agent: WWWOFFLE 
User-agent: Xaldon WebSpider 
User-agent: Zeus
Disallow: /