While technology brings many changes, some things NEVER change. Like the nature of people. The following is a curse found written by the doors of a 16th century monastery library in Barcelona, Spain:
“For him that stealeth, or borroweth and returneth not, this book from its owner, let it change into a serpent in his hand & rend him. Let him be struck with palsy, & all his members blasted. Let him languish in pain crying aloud for mercy, & let there be no surcease to his agony till he sing in dissolution. Let bookworms gnaw his entrails in token of the Worm that dieth not, & when at last he goeth to his final punishment, let the flames of Hell consume him forever.”
What DOES change is how easy it is to FIND something like this. I’d read this curse somewhere many years ago, but thanks to the ubiquity of information on the Internet, I was able to use Google and eventually find it again. Of course, that was after refining my search quite a bit. My final search was: library curse century medieval spain stealeth, after I remembered that the word “stealeth” was in it. 
Once I added “stealeth,” I got it focused right down to actual quotes of the curse. One good website that had this and other quotes, as well as a discussion of book thieves, was Museum Security Network BOOK THEFTS, which indicated that this quote came from an article in the Harvard Magazine.