I'm sorry, I laughed out loud at the image of the guests running amok after deleting the entrance.
I don't think anyone has come up with a "No Guests" hack.
I recently decided to demolish a zoo that had been started by someone else, so I couldn't just "restart" from the beginning.
What I did was name a male guest Alfred H. This causes birds to chase the guests screaming from the zoo- like in Alfred Hitchcocks movie "The Birds". I find this amusing.
I sold all the animals, deleted their enclosures, raised the entrance fee to $100.
After the guests stopped running and screaming, there were still a few, so I named another guest Afred H & that took care of the few stragglers.
Since I no longer have any animals in the zoo, no more guests will come in.
At this point, I will plan & build the entire zoo without placing any animals. I usually do this on pause during scenarios in order to not waste any more time than necessary.
If I'm doing a scenario that gives me animals, so I don't have a choice but to deal with them and guests, raising the admission to $100 causes fewer guests to enter. Then, I still try to do as much as possible on pause.