mod_defensible is an Apache 2.x module intended to block spammers using DNSBL servers.
It will look at the client IP and check it in one or several DNSBL servers and return a 403 Forbidden page to the client.
You can download it from the Git repository.
You can use =--with-udns= to enable udns (asynchronous resolver library) usage, which should be faster.
However, this is not mandatory.
# Activate DNSBL usage
# Specify DNSBL servers
DnsblServers xbl-bl.spamhaus.org. mydnbl.server.org.
With udns support, you can also use:
# Specify another nameserver to use instead of default system resolver
mod_ d efe ns i bl e