If you really dont want people mooching your well paid for internet, and not encrypt, you can just changed the ssid (to kick that guy off) and then stop broadcasting your network. That will stop the average user to gain access to your network. It most cases, it will never show up on a network list. The exception that ive seen, user on vista will see it, but they have to know the ssid. Now in no way is that completely secure. I just doubt that anybody that has the know how, to get your ssid from the air by stiffing all possible traffic will be around a housing community. It is possible though.
Ive had my network unencrypted for 5 years and never had one person hop on with my ssid not broadcasting.
I also see you never configured the router. Another "linksys" ssid out there. Its so common, its quite sad. I say log in by going to any web browser and type 192.168.1.1 as the web address. user name should be blank. Password is admin. 3 thing you should change. 1. the router password 2. your ssid, can be anything rememberable, and dont broadcast it. 3. your channel to channel 1. Channel six is over used and could vreate slowdowns with all the interference from everyone else that dont configure their router as well. If you want to encrypt, that'll be up to you