You can add the following list in the User Agents Blacklist in order to block a vast majority of bots:
Googlebot
bot
crawl
slurp
spider
mediapartners
You can find the list going dashboard > Downloads > Settings > Advanced > Access > Blacklist user agents. The last one will still allow visitors who enter fake email addresses to download your file or, if the download link is grabbed, to download files without subscribing.
