Spam and ghost referrers appearing in Google Analytics has been a major issue for some time. Your stats will be biased to some degree if there are lots of spam referrers.
If you want to know more about this and how complex it is to resolve, have a look at the article at http://help.analyticsedge.com/spam-filter/definitive-guide-to-removing-google-analytics-spam/
The first part of the article tells you about the issue in reasonably readable terms
, the second part gets technical and tells you how to try to fix it.
One takeout from this article is that Google says they are trying to fix it. But of course won’t say what they are actually doing because that would give the secrets away to the spammers. The spam seems to be getting worse all the time so whatever they are doing isn’t having much impact as yet.
I don’t see an easy resolution any time soon. Sites with small amounts of traffic are generally relatively more affected than sites with lots of traffic.