Block direct IP access from internet

First of all there is a simple rule: Use LTM Policy instead of iRule wherever its possible. Small benefit of this is less cpu and easier understanding. Simple create a LTM Policy with rules like

"if hostname xyz.com forward to pool 123" 

"if hostname abc.com AND tcp address matches 1.2.3.4 forward to pool 456" 

Do this with every hostname, apply this policy to your virtual server.