Domain name access only

OK. I meant that you should replace with the FQDN that your application is using.

I guess you are asking for an iRule that stops any requests with an IP address as the Host header.

when HTTP_REQUEST {
    if {[regexp {^(?:(\d{1,2})|(1\d{2})|(2[0-4]\d)|(25[0-5]))(?:\.((\d{1,2})|(1\d{2})|(2[0-4]\d)|(25[0-5]))){3}$} [HTTP::host]]} {
        reset
    }
} 

Note that regex is not very performant but this will check the host header against a regex for an IP address. You could consider using string match or the suggestion above by Stanislav if you know the domain that you want to use.