MacOS – How to block theself from time-eating websites

You can use an Extension like StayFocusd. Even Adblock works fine, really.

If you remember to always lock the PC when you leave it, you won't have this issue (assuming that you have a password) as the user won't know the password... But let's be honest, if there is something worth getting on your PC then tools can hack the password. So you'd need something sophisticated which will encrypt the entire machine if it detects this type of entry. But this is probably overkill I guess by the tone of your question.

An option is you can lock Google Chrome http://lifehacker.com/lock-google-chrome-with-built-in-password-protection-1601863648 but again, it's not a definitive way to keep some one out. I'm not suggesting that is the right tool, just giving an example that these types of tools exist!

However, if you're just talking about "general users" who could see this detail by mistake (and not espionage :) ) then the lock tool may just suffice, but it requires you to remember to lock it... If you can remember to lock Chrome, you can surely remember to lock the OS instead!

Of course, locking Chrome doesn't stop them installing another browser/version of that browser (depending on browser) etc and accessing the site another way!

Maybe a time out on the OS is the best way