Avoid Bad Neighbourhoods
What Is A Bad Neighbourhood?
A bad neighbourhood is any website or collection of related websites that the search engines consider to be using dubious techniques to manipulate their position within the search results.
How Do I Avoid Bad Neighbourhoods?
The simple answer is by avoiding having outbound links on your site then point at a bad neighbourhood.
If the decision on whether your site was part of a bad neighbourhood was down to which sites linked to you then any of your competitors could get your site run out of the search results by throwing up a lot of spammy sites and placing links on them to your site.
For this reason, the decision is down to outbound links because outbound links are considered by the search engines to be under your control. Either you place them there, or, in the case of community sites, your users place them there in which case you normally have the facility to moderate or remove them.
In the case of a community site where you do not have the time to moderate outbound links placed by your users then you should use software that will place a nofollow attribute on the link. If you are responsible for your own code or wish, by viewing the source code of your pages, to see whether the software you are using uses nofollow then a nofollow-ed link should look like...
<a href="http://some-dodgy-site.tld/page" rel="nofollow">Link text</a>
