I used to have same problem with my WP integrated Q2A site, and I found a simple solution for it.
I used cloudflare's free CDNs for about a month and it ended all the spams and when I stopped using it after first month spammers never came back. it's like those spammer bots added my site to their black list :D
it's free and it will speed up your site if you already don't have an excellent server, so I suggest it for both purposes.