I use a public library with reasonable frequency and have dealt with a similar issue in the past. I've found the most effective technique to be one that actually relies upon the fact that others are reading my screen.
I realise that I'm taking a gamble when I employ it, but in some half a dozen cases that I've pushed back, am yet to receive any negative repercussions nor has there been any continuation of the offensive behaviour.
I have a series of word documents that I choose between to open right before I head out for a cigarette, with my computer set to lock itself after 1 minute. I rely on the fact the place is covered with cameras and filled with people to prevent vandalism or interference with my laptop.
I simply address the behaviour and the words
"and stop reading my f*****g screen" at the bottom of each page. Whenever I return, I find that either the offenders have relocated or refuse to look at me or my machine any more. Sometimes they seem positively fearful of what I may do to them - that someone apparently unhinged enough to make an open threat as I do will actually carry it out. (I'm almost certainly a bigger pussy-cat than any of them - but the impressions one gives helps win poker..)
Something like the tactic employed by people that commit crimes against drug-dealers. I mean what are they going to do? Go and complain to library staff that some fella left his pc for 5 minutes with a threatening message on the screen addressed to them. :laughs:
To complain requires self incrimination...
Another tactic is one I've used for selfish people on forums or cheaters in tests - I give an answer that's purposefully wrong, but only ever so slightly and as subtly as possible. I hope that my 'work' is copied and causes pain to those that would seek to take unfair advantage of it.
Sure, they're both a little work - but each are effective at dealing with the problem and my frustration response towards it.