If they are accessing the IP through Squid, use a Squid ACL, like:
Code:
acl printer dst 10.0.0.5/32
http_access deny printer
...but if they're going through the firewall/router, you'll need to use the FORWARD chain, like:
Code:
iptables -I FORWARD -d 10.0.0.5 -j DROP
...and really, you might wanna do both.