We've been getting some spam postings recently. To counter these I have enabled "Image Verification for Posting". This means when you make a post you will need to view some letters in an image and type them in a box next to the image.
From what I have experienced there will be 4 characters to type and they could be numbers, letters or symbols (although so far I have only seen uppercase letters and numbers). You have a couple of tries to get it right.
Shouldn't be too onerous, but will prevent the spam.