YMMV whether this will fly in your company culture, but I titled mine "Focus time, please ask before scheduling".

And when people inevitably didn't ask, I'd just decline unless I especially wanted to attend. I find myself getting invited to meetings sometimes just because the organizer wants to be inclusive and make sure everyone is looped in who might want to be, and I figure that's what's going on if they added me without asking.

If it's really important for me to be there, they'll see my time block and ask me.

Google Cal will auto-decline meetings that occur during ‘out of office’ events.

I have several of these ‘out of office’ events recurring in my calendar that I use for concentrated work

To make them stick you have to decline meetings scheduled during that time