Some Draytek routers (the 2800 does, I think the 2820 does as well) have a facility which allow wireless connections to connect to the internet, but are prevented from accessing PCs on the wired LAN side. This is based on Mac filtering.
I haven't tested it to see if a wirelessly connected computers that are restricted in this way can see each other or not. (I suspect not).
You may want to consider a privacy protection solution such as WPA2, (and which also provides a degree of authentication)
I used this when my children's friends used to bring laptops round to do work and needed to use the internet, but I didn't want them to see the other machines on the network. (Just in case nightkhaos was wondering!
)