About i-Rule active_members...

You do not need an iRule for this. You can select 'Action On Service Down' to 'Reject' under the pool configuration.

That way when the pool is down (active members < 1 ) it will reject subsequent requests

Alternatively, if you really want to use an iRule, you can retrieve the default pool assigned to the VIP with

[LB::server pool]

when CLIENT_ACCEPTED { 
    set default_pool [LB::server pool]
    if {[active_members $default_pool] < 1}{ 
        reject 
    }
}