What can be done about a professor who leaves lectures too early?

As @Nate Eldredge this, how this is received is going to vary widely based on academic culture.

Let me assume that this is a course at a US university, and that it is an "official" course: that is, one that students register for, receive a grade for at the end, and the instructor gets some credit for teaching. In this context, the practice would probably not be acceptable to many key people in the university. At most US universities there is a notion of credit hours: there is a positive number of "credits" a student gets for a course (most frequently 1-4) and also a number of hours per week that the course meets. Now (by widely agreed upon convention rather than cosmic truth, of course) there is a fixed conversion factor for this: at my university, 50 minutes of class time per week = 1 credit.

An instructor who only meets for one hour out of a scheduled two hours is risking the disapproval of the registrar, other departmental faculty including the head, and the students of the course. The registrar is probably going to be unhappy because it looks like the course is trying to cheat the system: if everyone decided to claim twice the number of credit hours that they actually met, it would cause absolute chaos for (especially undergraduate) programs. The departmental faculty are also going to be unhappy (to say the least) about this, on the one hand because the number of credit hours usually figures into the teaching load -- the standard departmental teaching load basically amounts to faculty teaching so-and-so many hours per week (on average; loads can vary from one semester to the next). Claiming twice as much as you're doing is certainly not okay. And of course, most faculty want courses to be taught well, and it is hard for me to think of clearer evidence that my colleague is not doing a good job with his teaching than that he is only meeting for half the scheduled time. Finally, it is likely that the students are going to feel shortchanged ("halfchanged"?), and in light of all the stuff above, I don't see a satisfactory rejoinder to a student who feels that way: on the contrary, I think they're right.

So what to do? I would suggest beginning by bringing it up to a departmental faculty member. Good places to start are: (i) the instructor of the course, if you feel comfortable, (ii) the department head, (iii) whichever faculty member you feel closest to in the department (e.g. your academic advisor). For such conversations, it is important to stay calm and factual. Be willing to proceed in several steps. Definitely indicate an awareness that there seems to be some kind of real mismatch here, and if necessary you might want to mention that university administrators like the registrar might be concerned about it as well. (Again, I honestly think they would, and departmental faculty should know that they would. So this is more like a reality check than a threat of escalation.)

Another idea: can you ask some other faculty member (or person in a position of authority) to visit the course? Anyone who shows up is certainly going to notice a two hour course ending after only one hour. On the other hand, they may well talk to the instructor before they show up, which might result in the instructor's taking the full class time. Which is what you want, of course. If that happens once and then he goes back to halftime, then it gives you an easy opening to speak to him about what's going on.

I want to end by saying that this really is cultural. What you're describing sounds like quite an extreme case, but there is variation between US universities and personnel at those universities on the practice of ending early. It is not unheard of for instructors and students to feel that they have done all they need to do in a certain course meeting and thus end early: they may even feel great about doing so. For me personally, over the course of almost two decades of instruction at US universities, I have moved steadily in the direction of thinking that for a three credit course, the very least I owe the students is 150 minutes a week of me in the classroom. I remember though that I taught one undergraduate course at my institution, pretty soon after I got there, and for various reasons I decided to cancel three of the 50 minute class meetings, including the last one (excluding the final exam and review sessions for the final), because I was interested in presentations given by graduate students in a different course that met at the same time. The present me finds the past me's behavior slightly outrageous...So indeed, some variation on how this is perceived exists!