The simplest and best solution would be "Using whatever firewall you happen to be using". Just block access to port 22 from 192.168.8.12 (or specifically only allow it from 192.168.8.11).
If you don't currently have a firewall setup, then you may want to look into
iptables, which comes as part of the linux kernel. There's a tutorial here. There's also quite a few GUI interfaces for it.
The specific command you want to do what you've described above is:
# iptables -A INPUT -p tcp --dport ssh -s 192.168.8.12 -j DROP
This adds a rule to
INPUT for TCP traffic aimed at your
ssh port (22), that is coming from 192.168.8.12, and drops it.