Hi Ross,
Yes, you can block an IP from pop3 access similar to the way you would block it from smtp access.
Use the -x flag with tcpserver and create a rules file.
Here is the line of your pop3 supervise run file to update:
/usr/local/bin/tcpserver -v -R -H -c 500 -x /etc/tcprules.d/qmail-pop3d.cdb -l 0 0 110 /var/qmail/bin/qmail-popup \
And then create the rules cdb file /etc/tcprules.d/qmail-pop3d.cdb
If you need more specific directions for either of those steps please let me know.
-Raymond