irule rejecting all the request

We use something similar to the below to block access to some sites, unless you are aplpying the iRule to multiple VIP's or have multiple DNS names pointing to the same VIP I wouldn't bother looking in the host header it adds needless complication.

    set clientip [IP::client_addr]

if { [IP::addr 10.1.1.1/32 equals $clientip] } {

    pool pool_a
}
else {
    reject
}