[exim] How to ban some authenticated users?

Hello,

I have a problem about some users spreading their password to spammers.
So I'm searching for a way to ban some authenticated users, or at least
limit them to 1 message / hour with ratelimit.

But I can't find how to do it, because the many discussions about this
point to how to ban senders (that aren't usernames, but email address that
can be spoofed) or hosts (spammers change them every miute).

For example, is there a way to deny ldap authentication? If I modify the
'server_condition' line in this stanza:

login_server:
driver = plaintext
public_name = LOGIN
server_prompts = "Username:: : Password::"
server_condition = ${if ldapauth \
{user="uid=${quote_ldap_dn:$1},ou=people,dc=unimore,dc=it" \
pass=${quote:$2} \
ldaps:///}{yes}{no}}
server_set_id = $1

it will be reloaded at every login or only when exim starts?

Or is there a way to set a ratelimit for some users, and a ratelimit for
some other? And in case, if I create a list, will it be reloaded each time
or only when exim restarts?

Thanks in advance for the answer.

Fabio Ferrari