[TriLUG] iptables

[mark]

I am having a problem setting up some rules for my internal network - I want 
to block 2 of my systems from doing anything after a certain time at night.
I've tried the following but it didn't seem to have any affect:

iptables -t nat -A PREROUTING -s 192.168.2.33 -p tcp -d 0/0 -j DROP
iptables -t nat -A PREROUTING -s 192.168.2.33 -p udp -d 0/0 -j DROP

Even with the rule in place everything still seems to go by ...  If I missed 
something isn't the transition:

souce ---> Prerouting  ---> Forward ---> Postrouting ---> output
                        ^                                                  ^  
                      |___________________________|



Any help would be appreciated....

Thanks,
Mark