For a long time, I have been plagued by comment spam on my blog. To battle this problem, I have now installed a Movable Type plug-in that verifies that posters are not robots. This will hopefully improve the situation.
The plug-in is called scode. It displays a number in an image that the user needs to type to verify that he is not a robot. The plug-in was a bit of a pain to install: My version of the perl GD image creation library did not support PNG, so I had to use GIF, which took some time to find out. Also, scode required a fair amount of manual work to into the comment web-pages. But now it seems to be working. All is well.