How about interrupting at the same rate as a typical day in the office? That way the candidate can decide whether they really want to work there, and the interviewer can evaluate them under realistic working conditions.
Does adding comments in the code snippet help or stop this behavior? Like the first line says to the effect: "Please DND, Still working on code. Will reach out when done."
Pro tip for you. The interview isn't about writing a working solution, it's about communicating your thought process. You are focusing on the wrong thing if you don't want interruptions. As for "bonus" questions at end, again it's how do you deal with changing requirements, how well do you understand the problem etc..