[exim] Stop responding to bounces?

I've previously fixed the situation where the vacation message was
being sent multiple times. Now I'm having problems with a similar but
different situation

User A sets a vacation message
User A mailbox becomes full
Sender B sends a message, gets an autorespond from User A
Sender B's mailbox is full/invalid (such as spam) and their server
sends a bounce
User A receives bounce and sends a mailbox full bounce
Sender B server receives bounce and also sends back a mailbox full bounce...

I usually detect this after my server monitoring script reports
increasing loads above a certain level for more than 30 minutes
consecutively because of the sheer amount of bounces in the queue that
attempts redelivery.

After reading up, it seems that I can reject mails at the
acl_smtp_predata stage with a 550 error so that the message never gets
accepted for delivery and makes the sending server retry later for
valid senders.

However, it seems from sources found online that I cannot reject on
quota during predata because Exim doesn't know how big the message
will be so it cannot check against the quota. Is this still the case
or is there now a way to make Exim simply to reject the message once
the user is over quota without having to know the incoming message
size?