[Exim] Exim 3.36 - how are frozen messages deleted

I'm running Exim 3.36 with SpamAssassin 2.55 on RHLinux 8.0.

I have a system filter to freeze messages that are tagged as spam. It works
wonderfully.

My configuration has 'timeout_frozen_after' set to '7d'. Yet, there are
frozen messages that are 16 days old.

The online doc says that a "queue runner" should delete these, but it isn't
happening. Where should I look to find out why and fix it?

P.S. The online doc also says that bounce messages will be cancelled (just
deleted), whereas other messages will be bounced. I don't want anything
bounced, because it's spam and there's no point. Is there an option to say
"just cancel any and all frozen messages older than the timeout"? Perhaps I
should set the timeout to '0s' (to turn it off, right?), then knock together
a Perl script to periodically check for and delete messages?