IIRC, you could set a default policy of DROP on the output chain to prevent any packets leaving your PC, but this would be so restrictive it would break things like X. It would be far better to leave the output chain alone, and use a rule to drop outgoing packets on a specific interface, like this:-

iptables -A OUTPUT -o eth0 -d 0/0 -j DROP

The -o eth0 is used instead of -i because in this example the output interface is eth0 (packets leaving eth0 for all destinations will be dropped).