I spent a month as a security guard in a uni library, specifically to police everybody eating/drinking (water in closed bottles was allowed)/smoking. First time offense was ejection for the day; the second, for a week. The third (and yes, eating and drinking at the same time counted as two offenses) was an automatic fail for the semester (well, it was a written letter to the dean of your faculty informing them of this, and all deans had agreed to auto-fail those caught thrice).Sort of. The main issue with signs is kids eating in the library when there's big huge NO FOOD OR DRINK signs everywhere, even on the tables.
Computers actually aren't an issue most of the time, save the one time I caught a kid playing Runescape. They can't install stuff, so he must have brought it on an external drive or something.
Maybe a bit harsh, but there were very few people we had to write up a third time. And, you know, we tried to be reasonable. "handling old tomes" is different from "being at a study table with nothing on there but your own papers" (officially it wasn't) - we still wouldn't accept anything noisy or crumbly or with a possibility to stain, but still... On a table like that we'd probably give an oral warning and a "I'll be back in 10 minutes" thing. I actually did have to send someone out with a McDonalds hamburger while reading a book from before World War I, once. That guy came back and got caught with an open bottle of coke a few days later, and got his semester wasted.
Man, that was a fun time. Power! MUHAHAHAH! *cough* I mean, being able to do something about one of my biggest pet peeves.