Varnish analysis of content - do not cache conditionally based on content received

[pub crawler]

We have a situation where Varnish is caching things we don't want cached.

For instance,  sometimes we may throw an error and that ends up in the
page and stuck in Varnish.  We have common language / code we can say
will always be in such page.

Another example is we sometimes ban malicious folks as-needed on our
app servers.   When they get another page it simply says BANNED! #IP#.

Both conditions are able to be detected in theory if we can have
Varnish look at the pass-through page data it gets.

Anyone have any ideas of how best to accomplish this non-cache of
these conditions within Varnish?

Thanks!