Spam is unfortunately a problem which all WordPress sites with comments enabled will have to deal with at some point. I have spoken before about steps you can take to reduce spam but no system is full proof.
If your website becomes popular then high traffic pages will likely see a rise in spam comments. WordPress allows you to automatically switch comments off in posts after a set number of days. If you don’t want to do this then you may want to check out Ozh’ Spam Magnet Checker.
Spam Magnet lists the posts which spammers are most likely to comment on using popular keywords. This allows you to close comments on these posts.
Spam Magnet Checker is worth trying out if you have a medium to high traffic blog, particularly if you don’t automatically close comments.