Can anyone help me to stop spam comments?

We have started getting a lot more spam on our blog now. I have akismet installed and it is stopping hundreds of comments getting through, but there seems to be a new influx which I dont know how to stop as they dont follow a set rule.

They all have registered different email addresses, have different IP addresses and even ranges, they dont contain bad words it seems to be just gibberish like blocks of 8 random letters and the URL they specify is always different so I cant see a way of stopping this?

Can anyone help/does anyone else get these kinds of posts?

Thanks