You say no to them. I have a standing time reserved for school run. People regularly book stuff over it. I reject the meeting automatically. If I have 'busy' in my calendar because I don't want to be disturbed I first take a look what this new invite is and then decide on the spot with no or yes (no being the dominant response). Slowly over time people learn to email or Slack you with a few dates and times and life is good.
if you have time booked on your calendar, no matter what it is, it's safe to assume anything else put on top is going to be declined, because you already have something scheduled.
You can add a regular meeting for yourself too, instead of focus time, also set your calendar not to share meetings' details with other folks, so they will only see 'busy'