Modified Domain Cookie blocking

That looks like it should work. You could add logging to see what's happening. If it's a production virtual server, try adding the logging just for your client IP.

You could combine the two iRules so you only loop through the cookies once:

when CLIENT_ACCEPTED {

    Check if the client is our test IP
   if {[IP::addr [IP::client_addr] equals 1.2.3.4]}{
      if {$debug}{log local0. "Debug enabled for [IP::client_addr]"}
      set debug 1
   } else {
      set debug 0
   }
}
when HTTP_REQUEST {

   if {$debug}{log local0. "\[HTTP::cookie names\]: [HTTP::cookie names]"}

    Loop through each cookie by name
   foreach acookie [HTTP::cookie names] {

       Check if cookie starts with bcsi or _em_
      if {[string match -nocase "bcsi-*" $acookie] or [string match -nocase "_em_*" $acookie]}{

         if {$debug}{log local0. "Removing cookie $cookie: [HTTP::cookie value $cookie]"}

          Remove cookie
         HTTP::cookie remove $acookie
      }
   }
}

Aaron