Você pode usar ! -s or -d
para excluir alguns IP / Rede
Do iptables homem
[!] -s, --source address[/mask][,...]
Source specification. Address can be either a network name, a hostname, a
network IP address (with /mask), or a plain IP address. Hostnames will be
resolved once only, before the rule is submitted to the kernel. Please
note that specifying any name to be resolved with a remote query such as
DNS is a really bad idea. The mask can be either a network mask or a
plain number, specifying the number of 1's at the left side of the network
mask. Thus, a mask of 24 is equivalent to 255.255.255.0. A "!" argument
before the address specification inverts the sense of the address. The
flag --src is an alias for this option. Multiple addresses can be speciâ
fied, but this will expand to multiple rules (when adding with -A), or
will cause multiple rules to be deleted (with -D).
[!] -d, --destination address[/mask][,...]
Destination specification. See the description of the -s (source) flag
for a detailed description of the syntax. The flag --dst is an alias for
this option
Então, sua regra deve excluir o ip específico
iptables -t nat -A PREROUTING -i br0 '!' -d IPAddrOfyourRouter/32 -p tcp --dport 80 -j DNAT --to 218.108.168.73:82