Re: Next time
Short of tattooing the warning to the inside of people's eyelids, I'm not sure that any sign would work.
Many years ago we had to do a software upgrade to all the machines at a site. It was Windows NT (before AD) and yes, we had to go to each PC to do it.
Now this site had over 300 individuals - each of whom was a 'special case' whenever it came to any set of procedures -- you know the types.
The upgrade was planned for an overnight job and warnings sent out to all with a request to leave PC turned on, physically unlocked, but logged out.
A week before, posters were put up round the offices; A4 posters were printed and put in the kitchen, above urinals and on the back of toilet doors (to read when sat down) ... We even printed an A1 sized poster and put it on a trolley in the entrance foyer so people had to walk round it (not sure if H&S would approve...).
Some accused us of being a bit OTT.
Come the day, about half a dozen PCs were turned off and security devices applied and one guy who was working late wouldn't move; it was a 15-20 minute upgrade and he could easily have had a short tea break, but no - he was far too 'special' and 'important'.
Next morning, he and those who hadn't left their machines as requested couldn't log on. Their help desk tickets were noted but marked as minimum priority.
Cue much complaining about "crap IT, crap support".
Senior management were called in and started complaining about how much it was costing having these people unable to work and threats to call in HR as we were 'disruptive', 'uncooperative' and 'causing problems'.
Thank goodness for our manager who just said "Yes, let's go to HR right now. I've a few questions ... You're paying how much? ... for people who can't read or understand simple instructions that the vast majority managed to follow? ... or have project planning and management so tight that your guy couldn't take a tea break late in the evening? have you considered your recruitment policy for staff or planners?"
Sometimes you do get a good boss.
Biting the hand that feeds IT
