Given your rules, any client connection forward from LAN is allowed and subsequent (related,established) connections coming from Internet is allowed (forwarded) back to client. This is often typical or common of "simple" firewall/router configs I have observed.
Additionally, the -D option is to delete a rule from the given chain.
If for example, you only wish to allow LAN clients to access standard web ports (80,443), you might do something like this:
(going off the top of my head, but believe it to be accurate)
iptables -t nat -A POSTROUTING -o eth0 -j SNAT --to 124.xxx.yyy.zzz
iptables -P FORWARD DROP
iptables -A FORWARD -i eth0 -o eth1 -m state --state RELATED,ESTABLISHED -j ACCEPT
iptables -A FORWARD -i eth1 -o eth0 -s 192.168.1.0/24 -m tcp -p tcp -m multiport \
--dports 80,443 -j ACCEPT -m comment --comment "LAN Client to Web"
iptables -A FORWARD -i eth1 -o eth0 -s 192.168.1.0/24 -j REJECT
What is it exactly you wish to accomplish? And from where... Internet to LAN? LAN to Internet? LAN to router/firewall?