[exim] Prevent spamming to my server - Deny, Discard or Drop

Hi,

can anyone of you give me some advise about which of the ACL verbs are the
most convenient to prevents spamming to my server?

I think this:

DENY, if I use DENY, my server notifies to spammer that the recipients
address is not exists and the spammer may be to send more and more mails to
attack me.

DISCARD, if I use DISCARD, my server do not response anything to the spammer
and he/she thinks that the mail arrives OK and not disturbs more.

DROP, similar of deny.

Which you will use?

Thanks in advance.

Jorge Colaccini