If you have a lot of spam comments that you have collected over time and have not deleted, and you now wish to delete them, WordPress have a quick solution. Go into comments, click on spam comments, (these are comments that you would have marked as spam) You will see a link, that says Empty Spam. Click on it and they are all deleted.
Please remember, as you mark comments as spam, you should delete them there and then.
How to Bulk Remove Spam Comments in WordPress