Accessing Pool members directly

That depends on your network layout. Is the BIG-IP the only way to access the VLAN where the servers reside? If so, then the solution is already in place, because the BIG-IP doesn't forward any traffic that we haven't specifically allowed by way of creating a listener (Virtual Servers in most cases) for that traffic.

However if the server network is accessible through other devices such as routers/firewalls and so on, then you will also need to make sure that path is blocked.