Você queria (ver o !
):
iptables -A INPUT ! -s 166.100.102.50/32 -j LOG --log-level 7
Isso corresponderá a tudo com o endereço de origem NÃO 166.100.102.50.
De man iptables
[!] -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 num‐
ber, specifying the number of 1's at the left side of the net‐
work mask. Thus, a mask of 24 is equivalent to 255.255.255.0.
Aqui começa a parte relevante:
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 specified, but this will
expand to multiple rules (when adding with -A), or will cause
multiple rules to be deleted (with -D).