Well you can make a disk image with a password in disk utility. Go to Applications>Utilities>Disk Utility. Then at the top of that window, click New Image. Pick the name for your disk image and how big you want it to be with the settings shown in the drop down boxes. Leave the format at Read/Write, and if you want a password which you do, then set the encryion to AES-128. Click create and it will ask you to put in a password that will protect your disk image. Then once it's created, you have to put in a password to mount it.
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If you dont care about a password and just want to make it so you can't open the folder no matter what, lock it. Right click, or control+click and go to Get Info. Then under Ownership & Permission, click No Acess under the drop down box. If people don't know how to unlock folders, then this works great. When you want to open the folder again or add/remove items, just go back to Get Info and change it to Read/Write.