My website (and some other people's, hosted on my account) was down this morning.
Our web host suspended my account because trackback spammers pinged my trackback file too many times, causing the server's CPU to nearly overload.
Since its installation, Movable Type 3 has blocked 841 cases of trackback spam. In the last couple of days I received around 80 cases of such spam.
An analogy came to mind. Blog spam is annoying but inevitable (like mosquitoes in Singapore during the rainy season). I decided to let them be as long as most of it would be automatically blocked (using 'repellent') or deleted (relieving itches by applying balm).
However, when they start to affect the main system (spreading serious diseases like malaria or dengue fever), I am NOT happy.