May we never let the things we can't have, or don't have, or shouldn't have, spoil our enjoyment of the things we do have and can have. As we value our happiness let us not forget it, for one of the greatest lessons in life is learning to be happy without the things we cannot or should not have.
In particular, avoid links to web spammers or "bad neighborhoods" on the web, as your own ranking may be affected adversely by those links.
but with spammers getting better and better at exploiting sites for spam, sooner or later any site running with normal security may be exploited. If you're on shared hosting you may do everything to protect yourself just to experience that somebody else on the shared server had a hole they could exploit.
So what do we do to minimize the negative effect in Google of being exploited ?
I think if you stay away from it and make sure feedback is moderated to a certain extent on your site, there's not much to worry about. They're more concerned about people trying to game the system, and they want you to stay away from them.