You can block an user with an ip address from accessing your system using the iptables.
For that follow these steps.
1. Open up a shell and login as root.
2. now type the command below to block the ip(XXX.XXX.XXX.XXX).
# iptables -I INPUT -s XXX.XXX.XXX.XXX -j DROP
3. See whether the new rule has been added.
# /sbin/iptables -L -n
You can find other options to choose from with the following command:
# iptables --help