I have this with dss and it drives me crazy.
He has his own bathroom, and being a boy doesn't use my toiletries much (though I found one of my shave foams in his bathroom the other day and he takes disposable razors from our bathroom, usually mine as his dad uses a rechargeable thing).
But with him it is tech. He has to have his grubby (and they are bloody grubby all the time) hands on my stuff. Laptops, he just uses them (I have passwords on them, of course), mouse he'll just disconnect and take it if he thinks he needs it, chargers are the biggy - they are never where they are supposed to be which is so irritating as I like them to be in specific places I put things that will need to be charged. Last week I got a new Kindle. I have said he can have my old one (he doesn't 'need' it, he has loads of tech of his own, including a Kindle Fire) but when the new one came, I unboxed it, had a quick look, plugged it in to charge overnight. Went to work next day, came home and dss has been bloody messing about with it, changed settings, moved stuff around - I hadn't even looked at it! And yes, grubby greasy fingerprints all over it.
Meanwhile dp's Kindle has broken. He has moaned at me about it at least four million times. For the first two million I told him to moan at Amazon not me, you never know they might replace or fix it, but I can't do that. Now I just ignore him. Anyway, he now wants my old Kindle instead of dss.
But I cannot tell you how frustrating it is to sit at your PC to do some work only to find the mouse has disappeared and having to go round the house trying to find it/one to use.
And the stapler. Similar though not quite so debilitating re getting on with stuff. Oh and he is obsessed with Sellotape, he takes it and carries it around in this rucksack which is permattached to his back. I found two rolls in there the other day and can never find any in the house.
I don't really know what you do, except keep telling them not to take your stuff. Put labels on things?
I know that buying their own doesn't really make much difference. I also know it's not so much a 'step' thing as a 'kid' thing - but you feel so powerless when it's a step, you can't say anything without always feeling like the bad guy. And dss pretty much just ignores me anyway cos it's been like this for years and never changes. dp not interested in telling him not to do it - think he thinks I overreact to it.