Ipv6 is a superset of ipv4, meaning that every ipv4 address corresponds to an ipv6 address, but there are far more ipv6 addresses than can ever be represented by ipv4.
Spam is a problem independent of the protocol used to send it. If you want less of it, encourage a society that does not incentivize such profit seeking behavior.
If IPv6 addresses are a subset of IPv4s, then spammy IPv4 ranges can be blocked without having to look into an insanely large database at every possible spam query.
But I notice by your answer where you say it's not the fault of the technology that this may not be the case. If so then it would be terrible news and allow for tons of spam.
There are actually several ways to encode an IPv4 address as an IPv6 address, though only one--6to4--is in common use. This version associates a /48 IPv6 subnet with each 32-bit public IPv4 address. To filter these you would extract the IPv4 address and look it up in a normal IPv4 database.
However, most IPv6 addresses will not correspond to any IPv4 address. Only networks which have been assigned an increasingly scarce public IPv4 address can use 6to4 as a substitute for native IPv6, and the remaining IPv6 addresses have no connection to the IPv4 namespace. However, each end user is to be assigned a single, contiguous subnet, ranging from a /48 to a /64. Tracking these subnet prefixes in a database for filtering should be no more difficult than tracking individual public IPv4 addresses, and less prone to false positives due to address sharing (dynamic IPs and NAT). The databases would be somewhat larger, due to the longer addresses and more specific targeting, but not unmanageably so.
