Google's -30 penalty

Webmasters in online forums have discovered a new Google filter that Google uses to downrank web sites in its result pages. Has this filter been applied to your web site? How can you avoid that Google applies that filter to your site? What is Google's -30 filter?

The -30 filter is a penalty that Google applies to web sites that use spammy SEO techniques. If Google applies the penalty to a web site, the rankings of that site are downgraded by 30 positions.
Many webmasters in online forums reported that they had top rankings on Google and now they cannot get beyond position 31 in Google. Some webmasters even have #31 rankings for all of their keywords on Google.

Why does Google apply the -30 filter to some web sites?

It seems that Google applies the -30 filter to web sites that use certain spamming techniques. It seems that the following techniques trigger the -30 filter on Google:
Guestbook spamming

If you try to get inbound links by spamming guestbooks and blogs then Google might apply the filter to your web site.

JavaScript redirectsJavaScript redirections might be misinterpreted as a spamming attempt. Better use 301 htaccess redirect if you must redirect URLs on your pages.

Doorway pagesGoogle doesn't like doorway pages. If you must use special landing pages for PPC ads and other ads, make sure that these pages cannot be spidered by Google and other search engines. You can use robots.txt to do that.

What can you do if Google applies the -30 filter to your site?

First of all, you should remove the spam elements from your web site. If you use JavaScript redirects or doorway pages, remove them from your site.
The -30 filter seems to be an automated filter. If you remove the spam factors from your site then you'll probably get your rankings back after some time. You can also send a reinclusion request to Google.
If nothing helps, then the only way to get reincluded in Google is to get a new domain name.

Google doesn't like spammers. If you want lasting results then you should focus on ethical search engine optimization methods.