If someone comes to your desk directly, you are supposed to stop what you are doing and give your attention to that person, even if you are working on something really urgent.
Refusing meeting requests is also not well seen unless you have another appointment, meetnaping happens all the time.
A lot has to do also with open space offices, where everybody is just sitting there and makes it even more awkward, and you are right there available to be interrupted.
Interruptions from persons on your own team are to be taken with even more priority, the modern open office is just a high-interruption environment and there is nothing to be done about it.
In the end it might be really a good thing, because if a person is blocked it walks to the desk of someone else and sometimes can be unblocked and the company as a whole goes forward.
Doing what his boss did of cutting another person off with a 'not now' is not something that his boss can get away with a lot of times, even if he is under pressure: its demoralizing, it just ends up creating a weird atmosphere.
One solution might be: one or two days of remote work a week for everybody.