How do I restrict users?
If you're only worried about other users being able to see your files, run chmod -R o-rwx /home/maria. It revokes all permissions to the world (including guest).
guest does not belong to the admin group, which means he cannot run commands as other users (like root) through sudo (OK)
As noted by Takkat, anyone who has physical access to the machine could gain access to the files. The only way to protect you from that is encrypting the drive. If you put a BIOS password on the machine, the visitor can take out the HDD from it.