Referral spam can alter your perception of how your website is performing. A large offender in this category is simple-share-buttons.com. The general effect this type of spam has on your data will show inflation to your bounce rate, decreased time per session, alter your new vs. returning users statistics and decrease your pages per session. Not things you want to see on your analytics report. Rather than freak out, I suggest adding a filter to your Google analytics to protect the view of your data.
This simple youtube video did the trick for me. It’s easy to follow and set up. Good luck