Is it posible to put this i-rule, to drop a hostname, instead an IP addr?:

I think I understand what you want : you want to block if the client hostname is some_hostname.

the problem is the client never send it's hostname, so the only solution is a reverse DNS lookup for every request...

such filter is not security and will cause performance issue.

your irule won't be able to do it.

the following one can do (not optimized solution)

when CLIENT_ACCEPTED {
    if { [RESOLV::lookup @8.8.8.8 -ptr [IP::client_addr]] equals some_hostname } {
        drop
    }
}