No, you ahve to do it through your firewall. There's not such blocking option, and for good reason, in DC++ itself.
The only firewalls known to have successfully blocked external connections to DC are Norton, ZoneAlarm and Kerio. Theres a guide to doing it norton at
http://home.btconnect.com/ManyHappyReturns/Norton.pdf but no guides for hte other firewalls as of yet. If anybody manages it on ANY firewall and feels they can produce a set of instructions, please send them to me and i can add them onto the wiki.
This does not work on the Windows Firewall (properly) as it doesn;t block enough connections. (Well, I'm prepared to be proved wrong, but I never got it to work)
It basically boils down to producing rules for DC++ in your firewall that block everything except 137.205.0.0-137.205.255.255 (or 137.205.0.0/16 depending on how you add rules), and possibly allowing 127.0.0.1 for the stunnel connection, although norton does this automatically.
EDIT: ZeeClone has dragged up a guide to doing it on ZoneAlarm from when he was on campus, and this is now posted at
http://home.btconnect.com/ManyHappyRetu ... eAlarm.pdf
Bear in mind that this effectively puts the resnet, and all associated viruses on a trusted Zone, so is a bit of a secutiry risk. If you find a better way of doing it (properly) in ZoneAlarm, pm one ofthe ops and I'll change the guide.