Problem(?) in comments

Samuel B thanks for plugins, but I don’t need to mark it as a spam, I just want to understood how they are posted url’s. May be better add some check system, like generate random hash, attach it to comment form, and chek it before add comment to DB, if not defined – deny, else – allow to write. This is just an idea, may be you know already existing plugin with similar functionality?

BTW, I have math comment spam protection plugin, but, somehow spammers skip em.