With PeerBlock you would not really be trying to block other Bitcoin users. PeerBlock is just a great application to use with any P2P application in addition to your regular firewall. You just have to make sure you select the appropriate block lists for your P2P activity. Hence why I suggested this application, not so much to try and block other Bitcoin users, but to block 'know bad' computers, perhaps run by individuals who might have interest in 'trying' to hack / steal a Bitcoin wallet.
For example, if you go to List Manager > Add Lists you could add 'hijacked' from http://www.iblocklist.com/lists.php
- this list includes 'know' hackers, bot-nets, scammers and spammers etc. I'm sure you can understand the benefits there. The Anti-P2P lists also block companies and organizations that scan P2P users, blocking these will again help increase a Bitcoin users overall anonymity and PC security, including that of the Bitcoin P2P network.
These lists are compiled by individuals and a community that has a real interest in protecting computer networks and P2P users.
I mentioned not blocking the Educational list, for example, there would be absolutely no point in a Bitcoin user blocking MIT !
Building P2P VPN into Bitcoin in the future would be immense ! ...