I’m getting flooded with Spam comments

Someone out there keeps posting spam comments to my blogs and Im getting like 10 a day.
They posts viagra, cialis, sex, porn, lesbian, and worse.

I mark all the messages as spam and delete them but just get new ones by tthe end of the day

I had to change my system to await moderation before comments are published, but I hate that.
Most of the big blogs dont have to do that, the users can post right away.

How can I keep the spammers away?
thanks