Create an Irule action based on username

I think this code will help you out. You may have to tweak it a little bit (especially the response messages). You can also simply call drop to drop the connection completely instead of returning a message if you wanted.

when HTTP_REQUEST {
     Internal users
    if { [IP::addr [IP::client_addr] equals "10.91.0.0/16"] } {
         Access allowed 
    } else {
        switch [string tolower [URI::query "?[HTTP::query]" "username"]] {
            "admin" {
                HTTP::respond 200 Content {Acccess denied for admin account on external access}
                return
            }
            "apple" {
                if { not ([IP::addr [IP::client_addr] equals "172.65.0.0/16"]) } {
                    HTTP::respond 200 Content {You are not on the proper subnet. Access denied.}
                    return
                }

                 Access allowed
            }
        }
    }
}