How do I prevent Mac search from searching files in other user accounts?

The macOS default file permissions should prevent access if the files are in any other folder except Public. But if the files are in the root of Account #2 home folder then the files are visible to other users. Although they are read only. Since Spotlight can see them and read the contents it will index them.

You can change the permissions on the Account Users home folders and that will prevent Spotlight from even reading the files.

The default permissions on Account #1 and Account #2 is an octal value of 0755 for each of the home folders.

/Users/Account #1

/Users/Account #2

Although Apple documents some of the permissions abilities here:

https://support.apple.com/guide/mac-help/change-permissions-for-files-folders-or-disks-mchlp1203/mac

There is more you can do from the command line. In fact, this change is one we apply at work to better secure the company Mac's. We change the octal permission from 0755 to 0700. This will remove the Read permission from the home folder and you can't even list the contents of that user unless you are that user or if you are an admin on the Mac and you use sudo to get there on the command line.

The command to change the home folder permissions:

sudo chmod 0700 /Users/<Account #1>

sudo chmod 0700 /Users/<Account #2>

To reverse it just

sudo chmod 0755 /Users/<Account #1>

sudo chmod 0755 /Users/<Account #2>

You can change the permissions on the Account Users home folders and that will prevent Spotlight from even reading the files.

That's a bad idea. You should not alter the permissions set for the home folder.

Just create subfolders in the home and set them to match the others.

Delete the Group and set everyone to no access.

Move the files out of the root level of the home folder into the new folders or the built-in folders.