Working on a new website I had to import a wordpress database. No matter what my dump from phpmyadmin was failing, looking deeper I realize that the blog had 70 Mo of spam comments.
Wondering if it was the same on my blog, it appears that I had more than two thousand spams !
Instead of going to comment listing and erase each spam page by page,
I use a plugin that did the job on less than a second :
Spam Comment cleaners
So amazed to had nothing else to do than just click a button, this plugin did my day !
You can even schedule deletion time interval.