Search engines prevent some websites suspected of spamming from ranking highly in the results by banning or penalizing them. These penalties may be automated algorithmically or manually applied.
If a site is penalized algorithmically the site may start ranking again after a certain period of time after the reason for being penalized is fixed. If a site is penalized manually the penalty may last an exceptionally long time or require contacting the search engine with a reinclusion request to remedy.
Some sites are also filtered for various reasons.
- Google -30 rank penalty – an example of a penalty