Quickest firewall: ipchains -P input DENY
No, seriously, it's not that Slackware makes things hard but rather that it doesn't make them easy. Most Slack users are not your standard GUI-config types but ppl who like to use the flexibility of the CLI to make the box do what they want. It's also considered to be the most UNIX-like distro, which makes some of us UNIX admins feel right at home
For what it's worth, my suggestion is to take the time to read some online HOWTOs and docs, as they tend to be easier to digest than some of the man pages. If you have the time, there's nothing to be lost by trying to learn something new, and experimentation is key. There's a good ipchains howto at
http://netfilter.samba.org/ipchains/HOWTO.html that may help steer you in the right direction. If not, I'm sure someone else here can steer you towards some sort of firewall-management package.