I'm not sure which OS version this was introduced but the option is on the console port with version 7.1.
Please choose from among the following options:
1. Network Settings and Tools
2. Create admin username and password
3. Display log/status
4. System Operations
5. Toggle password protection for the console (Off)
6. Create a Super Admin session.
7. System Snapshot
8. Reset allowed encryption strength for SSL
You wouldn't want to disable the console port even if you could. And, if you set a password, make sure you remember it!
The console is the last-resort way to manage the device. I've certainly had cases where the only control I had over the device was via the console, when the GUI was nonfunctional. I've also had situations where I've absolutely needed to get on the console to recover a device by rolling back from the current version. If you set a console password, and don't know it, you will end up with a very expensive brick. (I assume Juniper has a way to recover a device even in that circumstance, but I wouldn't be surprised if it didn't involve returning the device to them.)
So, if you are installing a device in an unsecured environment and concerned about tampering, set a console password. Also, you might want to check to see if a syslog message is generated when an admin logs on through the console, and trap that syslog message with some sort of syslog postprocessor like Splunk.