[exim] How to permanently reject backscatter?

I'm getting huge amount of lines like this into reject log:

H=kingmani2.tempdomainname.com [128.121.215.219]
F=<> temporarily rejected RCPT someone@mydomain:
lowest numbered MX record points to local host


These lines are obviously backscatter caused by 3rd party
spam with forged from-addresses.

Now, I'm wondering what part of exim.conf makes it
to *temporarily* rejet these messages? The same happens,
although there were a decent from address instead of
F=<>.
Actually, I'd like to change the response from temporary
to permanent reject, in both the cases: whether there is
a valid from address or not.

Now, I have in 'acl_check_rcpt:' the following:
1)
require verify        = sender

2)
accept  domains       = +local_domains
          endpass
          message       = unknown user
          verify        = recipient

Both above are from (some old) original exim.conf file, and
the comments say "otherwise deny". What this "deny" means,
is the temporary reject caused by these conf items? If so,
how can I change it from temporary to permanent reject?


Regards,
Timo