You can change the folder's permissions so no user other than root can access the files inside:
# chmod 600 /path/to/folder
Then set the owner as root (if not already).
# chown root:root /path/to/folder
This will set your folder so that all user accounts on the local machine are denied access to the folders contents, except for the root account.
I'm not sure what you mean by password protecting a folder without encrypting it. There wouldn't be much point of "protecting" a folder with a password if it is not going to be encrypted in some way or another, assuming all accounts on the system have read access to the folder.