Hello, I received a message from Google about artificial or detected unnatural links. I didn't know why my site violated this policy from Google. My question is, How do I protect of artificial or detected unnatural links? I used to advertise my url to a forum, is that the cause? Thanks.
The disavow tool alone might not be enough. You should try to remove many as possible. Yes, spamming forums is the cause (or part of). Any spamming/inorganic linkbuilding can be the cause. Google's Webmaster Guidelines are easily accessible and published for many years, so you should know that your site violated policy. Spend some time reading them - ignorance isn't an excuse; Google will penalise irrespective. Also read this: The Death of Link Building and the Rebirth of Link Earning - Whiteboard Friday
forum posting will not be the reason of unnatural link unless that you are spamming. I suggest you to check your backlink, this kind of issue normally due to auto link builder or spamming link