Stopping visitors leaving trick is a Cheat/ trick that the player can use during Theme Park World gameplay where they block visitors from exiting the park in order to reap certain benefits/ easily achieve goals or Challenges the game have set.
Some of the benefits to preventing your visitors from leaving, whilst still allowing more to enter, is that the amount of people currently in your park raises without ever decreasing making it much easier to win challengers or Golden Tickets requirements that involve attracting large amounts of people to your park.
The visitors moods tend to go down if you they are trapped and unable to leave the park, be prepared to be in the low Blues in the meter. The park also tends to get very messy with the large amounts of visitors. Queues for things like your park's Rides and Shops can become very long resulting in more waiting and more unhappiness.
How to perform Edit
Start by buying small rocks from the features section of Purchasing screen and lay them out to block off the park entrance from the rest of the park. Choose a ride to become the only entrance to your park, it must a separate entrance and exit track. Place the entrance in the visitor side of the Rocks and the exit on the other side which will mean that when your visitors go on the ride they are "entering the real park. Then you just need to build the rest of your park after the rocks which your visitors can enjoy until you release them.
You may need to turn more than one ride into your park as otherwise there will be a large amount of people Queuing for the ride. To quicken the entrance rate, turn down the rides duration and the capacity up.