how can i make irule to block port 25 on virtual server?

Do you really require a Any port virtual server.

Starting with Exchange 2013, all exchange services are on HTTPS port.

if you require it, try this irule:

when CLIENT_ACCEPTED {
    if {  [TCP::local_port] == 25} {
        TCP::respond "500 Service not available, closing transmission channel"
     }
}