I have begun to hear recently that having backlinks from certain 'spam friendly' portals can be a very bad thing. Would using a robot.txt to keep these spiders out be effective, or would they just pull your data from your site anyways through other search engines?
I'm not quite sure what you're talking about but I'll try to answer what I guess you want to know. ;)
Having a robots.txt that prohibits real search engine spiders to go through your site mostly doesn't help to avoid backlinks. There are lots of bad bots out there masking as Mozilla or just not obeying the robots.txt. The only thing to handle this stuff is asking the site owners to remove the link. If they're stealing content, you can get the sites out by contacting the search engines.