For whatever this is worth (and I know this is a pain so I'm NOT suggesting this situation is acceptable) a workaround might be to copy the URL of the offending post from the offender's server's website and DM your server admin (and the admin of the offender's server if you haven't blocked it) directly with a message that includes the URL.

Another alternative is unblocking, reporting and then blocking. But again to be clear I'm not saying it's acceptable to be forced to do this.