iRule Syntax to Block IP Address in AFM

See K10354610: BIG-IP AFM operations guide | Chapter 3: Firewall rules and ACL::action for additional details.

when FLOW_INIT {
  if { [class match [IP::client_addr] equals DG] } {
     ACL::action drop
  }
}

Is there any reason you want to do this specifically within AFM as opposed to just matching the data group and dropping without ever involving AFM?

when FLOW_INIT {
  if { [class match [IP::client_addr] equals DG] } {
     drop
  }
}