While interruptions are bad for your own productivity, they may really boost the productivity of the person doing the interrupting. I'd rather have a junior dev interrupt me when he needs a quick answer to a question instead of spending an hour googling and copy-pasting a dangerous hack from Stack Overflow. What's the point of sitting in an office when you don't collaborate?

The trick I use to reduce the effect of interruptions is to not respond right away if I'm in the middle of something. I just say "I'm in the middle of something, let me just finish this!" and then I get back once I'm at a point where my "mental load" is reduced.