There is no delete spam directly option in akismet plugin by matt.I found one plugin which blocks spam so efficiently , it was suggested by matt cutts in world web conference in his presentation.
The plugin works on very smart idea.The plugin loads an extra CSS sheet with your theme.When a real human user comes to your blog the CSS get loaded.When robots comes they don’t load your whole site.They load only comment box.So the extra CSS doesn’t get loaded.The plugin detects comments without CSS and sends them to spam or directly deletes them as per your options.
The best option is it gives ability to delete spam directly instead of sending to spam box.Huge site will get help from this for sure.
It is notable that this plugin has given me 100% result up to now.
Download Link: http://wordpress.org/extend/plugins/cookies-for-comments/