You can do this by using a couple of variables or a switch.
At the end of the first level inside the last Event, add a switch or variable to indicate the level has been completed. With a switch, you turn it off or on so it's a bit limited. Using a variable, you can start at 0 and increment it by 1. The beauty of the variable is you can use it to represent the completion of multiple levels.
For your rock event, create another event page that triggers when either the switch is ON, or the 'level complete' variable is set to 1. On this Event page, you would leave the icon blank and set it Below the Characters and trigger by Action Buttion. Alternatively, you can insert an Erase Event if you needed it to reappear if the player leaves the map and reenters.
I do something similar for treasure chests. Until the player completes a quest/task - and sets the associated variable - the Conditional Branch in the treasure chest event won't open the chest.