Well, according to a quick search on the Internet it seems that you could include this option in your command line:
to completely disable ipv6, or
Code:
ipv6.disable_ipv6=1
to avoid assigning IPv6 addresses to any of your network devices.
To know more, see these documents in your Slackware:
/usr/src/linux/Documentation/kernel-parameters.txt
/usr/src/linux/Documentation/networking/ipv6.txt
PS blacklisting ipv6 work as well, because the IPV6 driver is configured as a module in Slackware kernels.