I thought I'd share how I did it in case it helps anyone else.
Us bloggers love to receive comments from our readers but this is the ultimate way to put people off from commenting: I should know because I often don't go through with posting a comment when presented with the captcha unless I really really like the blog, I simply have much better uses of time than learning sandscript or whatever it is.
Here's how to remove it from your Blogger blog (you'll can still choose to approve comments before they are published so you won't get spammed into oblivion).
Go to your normal blogger dashboard (by clicking on the Blogger bar at the top of your homepage on the right hand side and selecting 'design'.
Why don't you try it out and leave a comment and see how easy it is?:)