Use built-in OS permissions to restrict access
On Windows, right-click the folder, open Properties, then Security, and remove or deny unwanted users
On macOS, right-click the folder, choose Get Info, then adjust Sharing & Permissions
On Linux, use chmod to change folder permissions
On Linux, use chown to change folder ownership
Encrypt the folder with a password-protected archive
Use a third-party folder-locking app
Store the folder inside an encrypted drive or container
Set a strong account password on your device
Enable full-disk encryption on your device
Back up important files before changing permissions