Use a man. And manhandle them...
No, just follow the regular procedures. A protest zone is always a good idea, but what they need is what virtually all campaigns have, which is security that restricts entrance, searches people to make sure they have no prohibited items (such as campaign literature from other campaigns, banners, etc.), and an efficient off-duty police security force to remove people who have slipped in and start to disrupt the proceedings.
This is standard at all campaigns; I worked for GWB in Florida, and I saw it in place there, and the Dems had the same thing. Either Trump’s campaign managers are idiots, or he believes permitting these attacks will get him more publicity and more supporters.