You have 2 choices if you want to lock down a folder from other users
Encrypt or create a compressed password protect archive of the files. The methods in https://askubuntu.com/a/104545/25863 and https://askubuntu.com/a/104984/25863 are perfect for this case when you need to be 100% sure no one will ever look at your files without knowing the password used there.
Use your computer folder / file permissions to deny access to your folder to other user.
The fist involves making sure that the result cannot be opened by any user without knowing the password used.
The second will only change the file / folder privileges so that another user without rights cannot open it. The folder still exists, can be accessed by any user with sudo rights in your system or using a LiveCD and reading the partition. It is also simpler to implement and does not require that you type a password all the time you need to open / mount the encrypted folder / archive.
An easy and fast way to do so is using
chmod to change the privileges for a file or folder.
Open a terminal and navigate to the place where your folder is, lets assume that the folder name is
foo and that we are currently located where the folder is.
chmod 700 foo
foo only available for your eyes, its not encrypted (that is also possible) but only your user (and or a user with sudo privileges) can read or open that file / folder.
You can also do it graphically by right clicking on a file or folder and changing its permissions manually. In the example bellow you as owner have all privileges and you deny any other group or user from accessing, reading or modifying that file / folder
For more information about file / folder permissions visit the Ubuntu
Understanding and Using File Permissions wiki page.