Query about sharing/privacy

Hi guys, I just noticed someone's computer name (I think it's my neighbors) under the "Shared" section in Finder. Is this person using my wifi? Should I be concerned? Can I block them?
Thanks in advance.

Erm, I miss read that..


They're on the same network as yours. Use WPA, use a hard password. Do so now.

I will do that. Thanks for your help!