[Exim] How to disable specific entries in aliasfile?

Hello.

I have a virtual domain setup similar to the one in FAQ/Q0401.
In addition, I want to disable some accounts in this aliasfile, which have
been used for spamming.

The aliasfile looks like:
one:test@???
!spam:
*:postmaster@???

I tried to use a condition with a lookup on !${local_part}:
condition = ${lookup{!$local_part}aliasfile{0}{1}}

It did not work. The lookup succeeded (exim -d9: "director skipped,
condition failure"), but afterwards, exim resolved the * default receiver.

Does anyone have an idea how to make the account "spam" disabled?

The ! is just an idea, it could be done in any other way, too.

Aliasing it to "trash" (assumed that trash gets delivered to /dev/null) is
no good idea, since it wastes bandwidth. Preferrably the message gets
denied at SMTP-time with any 5xx code.

Kind regards,
Volker