Referral Spam occurs when bots visit your website, and these visits are then recorded in Google Analytics with the spammed URL attached.
These URLS could show up as:
These crawlers generally do not make your site unstable. Therefore it is not necessary to block them via Savvii.
Using a filter in Google Analytics usually suffices when trying to prevent referral spam. In the article below you will find many different ways to accomplish this.
Unfortunately, you will be fighting an endless war against spammers since they will continuously change their domains.