Some web hosts do not allow for plugins such as Akismet to function properly. This is because they turn off the PHP function fsockopen for security reasons. When they do this, any plugin that connects to an outside source doesn’t function correctly. The biggest issue then is the inflow of spam.
I’ve found two plugins that seem to help take control of spam comments and trackbacks when Akismet isn’t available. Neither are perfect, but together they make a world of difference in the fight against spam.
The first is Cookies for Comments. This plugin places a cookie into the visitors browser when they arrive at your site. Once they leave a comment, the plugin checks to see if that cookie exists. If there is no cookie, then it’s marked as spam. From my testing, it works quite well.