There are various things at play here. First, I get the feeling that most of the people who have answered come from a corporate environment where the workplace is an office building with a receptionist and a waiting area. I have personally never worked in such a place, so it isn't what I consider the norm. Instead, my experience is from academic laboratories and small tech startups. Neither of these necessarily (or even often, in my experience) have a receptionist or waiting area.
So, if I'm working in my lab back when I was in Academia, or in our single room, open-plan office in my current job, if someone comes for our interview an hour early, I need to drop everything and go to baby sit them. Or, I can tell them to wait and then go back to my work trying to ignore the fact that some poor sap is waiting two meters away from me staring at me while they wait for the time to pass.
This is not a pleasant situation. Sure, if you work in a big place with loads of people, a cafeteria, a receptionist and all the works, it isn't such a big deal if people arrive early. At least, you don't need to deal with them and they have somewhere to wait until the right time. But this is not always the case at all. Many workplaces that don't rely on or expect interaction with the public do not have such spaces and arriving early means somebody will have to stop their work, break their concentration, skip the meetings they had planned for this time and deal with you because you were not capable of keeping an appointment and instead arrived an hour early.
The next issue here is one of punctuality. If I tell you I will meet you at 4pm, I mean 4pm. I can live with 10 or even 15 minutes in either direction, but an hour early or an hour late is crossing a line. If you arrive an hour early and don't even bother to call ahead to warn me, don't apologize, don't offer to wait elsewhere but instead waltz straight up to my office, ring the bell and expect to be dealt with, then you are not coming across as very professional. Being on time means being on time: arriving at the agreed upon time. Being early is, by definition, a way of not being on time. And it can be just as disruptive as being late if my workplace isn't set up to accommodate people waiting.
If someone arrives an hour early, they should apologize. They can ask if it would be OK to wait. They can ask if it would be OK to move the meeting earlier. But they shouldn't just waltz in with as weak an excuse as "I finished lunch early so I came by" and expect to be waited on. That really comes across as very rude and presumptuous to me.
If I were in the OP's place, I would simply tell the person "I'm sorry, but our interview is scheduled in one hour from now." Then, if I happen to work at a place with a space for visitors, I would tell them to wait there until I finish whatever else I have on my plate or, if I don't have such an area I would ask them to go away and come back at the right time.
At the end of the day, in a professional setting, it is important to be punctual. And you can only be punctual if you arrive on time. This idea that being early is somehow a good thing is mystifying to me. These days, I have my working day planned down to the minute in many cases and if you pull me out of the zone or a meeting because you came in early I will not be looking at your application very favorably.