How do I delete a "deleted user" in Mac OS Monterey?

Open Terminal in /Applications/Terminal.

Type the following commands:

List the contents of the /Users folder:

ls -l /Users

Identify the precise users home folder you wish to remove e.g. "/Users/username Deleted"

Type the following to delete the precise user home folder, if there is a space in the name enclose it in quotes, enter your Mac password to execute the command. The sudo command means Super User Do and the rm command is remove (delete) and the -r is recursive meaning to delete the parent folder and all sub-folders and the -f is force so it doesn't ask you to confirm:

sudo rm -rf /Users/"username Deleted"

Deleting 350GB may take several minutes, let it run. When the cursor comes back and moves to the next line it has finished and you can type "exit" and press Return then quit Terminal.

You typed a numeric #1 instead of a lowercase L.

ls -l /Users/"Deleted Users"
sudo rm -rf /Users/"Deleted Users"

There's a couple of scenarios here depending on the choice made when deleting a user account:

The first option creates a folder /Users/"Deleted Users"/ and inside that folder are disk image files for each deleted user (DMG files). /Users/"Deleted Users"/test.dmg

The second option says not to change the home folder but it actually renames it from /Users/test to /Users/"test (Deleted"/

Notice how I am using double-quotes around directory or filenames with spaces. You can use single or double-quotes.

Valid ways of dealing with a space in a folder or filename when on the command line in Terminal:

/Users/"test (Deleted)"/
/Users/'test (Deleted)'/
/Users/"Deleted Users"/
/Users/'Deleted Users'/ 

As to referencing the "Gray Area" that must be the System side in About this Mac -> Storage? Well it's likely a large snapshot or multiple snapshots on APFS. Connect up Time Machine drive and let a backup occur a few times and it should reduce it.

In Terminal, you can obtain the human readable size of a folder thusly:

sudo du -h /Users/"Deleted Users"/
sudo du -h /Users/"test (Deleted)"/

Depending on the amount of data, deleting 350GB might take a long time. I doubt it would take 12 hours. Try a reboot and try deleting again. Or perhaps boot into Safe Mode by holding Shift at power on (Intel Macs) or Hold power button on M1 Macs and hold shift when mousing over the disks and there's a button below to choose Safe Mode. Then try deleting the user again. Safe Mode boots without any extra background services running. I know of several IT related tools that would slow down deleting and copying files as they need to scan every file. Security software doing that is one reason why it can slow down a Mac considerably. Antivirus, Data Loss Prevention, Malware blocking, etc. They all scan the drive and slow down file access when copying, moving, or deleting files.

I have a similar issue.

My original Administrator User (Greg) became corrupted and I created a new Administrator (Lawrence). The system seems to be working fine with using Lawrence. I then deleted "Greg" and the Disk Utility is still running after 12 hours saying that it's deleting the files.

I only keep applications on the iMac drive (500 GB) - Lightroom/Photoshop/MS office.

The finder shows that the various folders under deleted users/Greg are empty. The Trash is empty.

My concern is that the grey area within the Hard Drive is showing 450.5 GB, the amount used is 15.75 GB.

I'm a novice. Is there any danger that the grey area contains any files necessary going forward.

Can you confirm that the Terminal instruction should be:

ls -l /Users/"Deleted Users"

then, when prompted

sudo rm -rf /Users/"Greg"

I have a closely related problem. I have a deleted users file but the file is completely empty and the terminal command is showing only my current user. However, I cannot delete the "deleted user" folder even though I have updated and unlocked the permissions and it is completely empty. Any suggestions?

You can create an actual user and link the folder to the user. After just delete it by the book.