Damn freeloaders...

So I'm browsing the internet today, and I'm thinking damn is this thing slow. My brother isn't home, so his computer isn't on, my parents' computer isn't on, and I have zero network activity from my computer at the time, so I'm curious as to why the DSL modem is going nuts. I logged into my router to find out some jack*** is freeloading off my DSL! It showed all the users currently logged in, and I have specific names for all the computers in my house, but there's some computer with a random name of letters and numbers listed, and I know it's not one of mine. I never set the security on my wireless access point because it made networking across the wireless just crawl, so it's my own fault, but that just annoyed the hell out of me. My brother's computer is the only one on the wireless because I haven't felt like cutting up the drywall and pulling cable through the blizzard of blown insulation in my attic, but apparently one of my neighbors decided they'd use my DSL for free since I never turned on any encryption. I set the wireless to allow only my brother's computer to authenticate from his MAC address, but do you guys think this is enough, or should I turn on the WEP encryption, and just make my brother suffer through painfully slow networking?

Before my laptop died, I turned it on once, and it slowed my 802.11g connection down to 6Mbps from 54Mbps. I turned it off, and it went right back up. It didn't seem to make sense, so I did it again, and it had the same result, so I just left it off all this time (a couple years now). I should've messed with whoever it was freeloading off my internet, but I was too pi**ed that someone was doing it to check and see if he had any file sharing enabled, so I could delete everything except his Windows and program files directories. MAC authentication should serve the same purpose as WEP as far as being able to connect to the wireless, except that it doesn't encrypt anything, correct? I doubt it was someone spying on my internet usage, and instead just someone like "wow honey look, this wireless card picks up the internet when we don't even have it!"