Leaving won't stop immigrants unless the UK government changes it's policies to immigrants.
Eh?
As an EU member state we must allow free movement of peoples from other EU members states into the UK to live and work. We cannot turn them back at the border unless there is good reason. e.g. criminal record / suspected terrorist etc.
Leaving will mean we are not bound by this so this -will- stop immigration we have no control over.
Immigration will still happen but purely on our own terms.