What is the appropriate response to students who peek at the exam questions before the exam starts? I have been an invigilator for several exams for large classes. I am not formally affiliated with the classes.

One thing I routinely notice is that, despite warnings, there will always be some students who will very quickly and brazenly flip through the entire exam (booklet) and look at the questions before the exam starts. Some will do this very quickly to get a sense of the type of questions (no more than 10 seconds).

What should be the appropriate response to this type of behaviour? The dilemma for me is, I don't want to expel a student for a less than 10 second glance at an exam, but I have been very annoyed by this type of brazen behaviour (even when I was an undergrad), which clearly constitutes cheating. 

Can someone please provide advice as to what to do for this minor type of cheating behaviour?
I don't know what an "invigilator" is exactly, but I presume that you merely watch the students to make sure they don't cheat, distribute exams and paper, and take the filled exams at the end, more or less.

As such it isn't your role to decide on any kind of punishment for cheating. There are probably procedures in place to report cheating. Do it. What happens next isn't up to you. If there are no formal procedures, take the name of the student and inform whoever is in charge (the professor in charge of the course for example).

Unless you have explicitly been told that it is okay, I would recommend being extra careful about expelling students. Students are afforded due process. If it is ever found out that peeking at the exam isn't something worth getting a zero on the exam or if the student is not found guilty, then having expelled them is something that will be terribly hard to correct. In my university the rule is to expel students only if their behavior causes trouble for the other students or in cases of person substitution (someone is taking the exam in the student's place). And even then, it's not the role of the proctor to decide this -- you have to get approval from the university president's office.