By not allowing posts from new accounts for a time period
By not allowing multiple posts within XX minutes
By observing nicks of spammers and removing them (This can somewhat be scripted btw)

Anyways, that is not an excuse for blocking tor services at all.
We have also seen massive spams from certain countries, why are those countries not entirely blocked ? I will answer that myself. We dont want to harm friendly users from those counties.
Why do we want to harm friendly tor users then?

edit: aboy (D)DoS: https://www.torproject.org/docs/faq-abuse.html.en#DDoS