Gag SO, tie em up & suspend them in the air in the basement or attic..
Helps if SO is into BDSM..if not it's a bit awkward.. // joke, I don't even have an attic or basement.. :(

In all seriousnes, if anyone knows how to prevent people from knocking on the office door to ask if I want coffe or sth to eat, that'd be great..asking them to not disturb unless the house is on fire or they cut off their fingers clearly doesn't help.. :(

I'd go with the gagging option.

Spray them with a spray water bottle each time they interrupt you.
If that does not help, replace water with nastier substances. They will learn pretty fast.

It took me a while to realize this wasn't about StackOverflow. I should be concerned-

I've found that sitting them down 1-on-1 for the explicit purpose of making it clear to them that they need to leave me the hell alone works pretty well. Looking them straight in the eye and being serious helps.

It will take some time, as for some people acts of kindness are the way they express that they love you, and so each time they slip up, you gently remind them that you talked about this and you need solitude right now.

If they aren't getting it by the next month, sit them down again and remind them more firmly and sternly that you need this.

The third time you have to sit them down you shouldn't be asking. Be assertive and be honest about how it makes you feel when they repeatedly keep doing this without your expressed permission.

After that, start keeping a gun by your desk. Start with the fleshy bits and work your way up.

🚪👌 Hey do you want coffee or something to eat? 🤨

@hinst Yes! Bring over some kebab!! 🤣🤣🤣

@sladuled At first I thought, why would you want to gag Stack Overflow?! 🤣
You could try to put a red illuminated "ON AIR" sign outside or maybe some police line tape or "RADIATION LEAK" warning.

@PonySlaystation I intentionally don't write it out.. to see who is the geekest of them all.. ;)

Edit: also it doesn't work.. o.O