centos - Ensure PPTP / OpenVPN clients cannot interact with each other?

How can one ensure that PPTP / OpenVPN will not allow clients connected to the tunnels to be able to interact with each others?

I never enabled bridging and enabled the following in iptables

iptables -t nat -A POSTROUTING -s 10.8.0.0/24 -o eth0 -j MASQUERADE

iptables -A FORWARD -i tun0 -o eth0 -j ACCEPT
iptables -A FORWARD -i eth0 -o tun0 -j ACCEPT