* Don't install any company apps or log in to any company accounts on your phone.
* Set your working hours in your calendar and stick to them.
* Set your Slack status to automatically show you as away and snoozed outside your working hours.
* When you're done with work, shut down your laptop and walk away.
I've never received any complaints from sticking to this pattern. When I'm in my working hours I'm consistently reachable and I do my job, when I'm not you can't get hold of me if you try. This is how it was in the office, I don't see why the expectation should be different in WFH.
I do think that a key thing that makes this work well for me is that I'm consistently online during (and only during) my specified hours, rather than mixing and matching my schedule on a day to day basis. I'm not always reachable, but I'm predictably unreachable.
They now stuck to their hours, got used to saying no, but worked hard while they were 'at work'. Part of the thinking was this moved them out of the group who were always staying on to finish something off, which can make you look ill-prepared or behind - despite the fact that you only there are you are taking on additional work.