problem to block customize ip

[hike]

i installed openvpn server for my centos 7 server and it is working fine . but i want to configure it using iptables to customize the LAN access behind the openvpn server.
I used iptables nat configuration like

iptables - t nat -A POSTROUTING -s <vpnserverip subnet> -o <hostinterface> -j MASQUERADE

as result it will access all LAN host behind the openvpn server...but i want to access particular host

how it does ..pls help