As things go forward, Trump can use his hospitality-management experience to make all rallies closed except to registered donor/ticketholders only, with small side rooms containing closed-circuit television and snacks for others. Any groups who arrive with signs, matching t-shirts, backpacks or large cameras can be broken up into the small side rooms by the gatekeepers, and never let into the main auditorium.
And if physical confrontation cannot be avoided?