A lot of illegal immigration will be stopped by changing the mindset of the would be illegal immigrant. Wall too high? Payoff too low once they get here? Many factors will go into it.
The wall is just one factor. The wall will physically turn some back because it is a physical obstacle. We've already seen that happen, and we don't need an expert to testify about the commonly observed fact. But perhaps more importantly, the wall will stop some people from making the journey in the first place. They know about the wall, so they will not come, perhaps because of the wall itself as a physical obstacle, but perhaps more likely because spending that money on the wall indicates that the US is serious about this issue, and therefore the US will likely take other actions as well when they get there. The wall is a big billboard which effectively says, "Don't try it. It's not worth it."
But I think the most effective thing is what Clinton did, which is pursue companies that employ illegal immigrants.