Or, if that is not an option, by creating an ip6tables rule:
ip6tables -A PREROUTING -t mangle -p icmpv6 --icmpv6-type neighbor-solicitation -i `nvram get wan0_ifname` -d ff02::1:ff00:0/104 -j DROP
Or you can experiment with the ARP cache limits: sysctl net.ipv6.neigh.default.gc_thresh1=256
sysctl net.ipv6.neigh.default.gc_thresh2=512
sysctl net.ipv6.neigh.default.gc_thresh3=1024
[1] http://serverfault.com/a/461053