How easy is it for a competitor to ruin your rankings? It is not very easy, but also not entirely impossible. Google declared several years ago that certain abuses would not affect the ranking of page. Instead, it would be ignored, so there would be no benefit. Negative SEO is an activity that one competitor engages in to lower a competitors ranking. If you have a sudden drop in rankings, this could be one of the potential causes. Here are some of the more problematic negative SEO tactics and how to deal with them:
Anchor text spam
There are groups that run link farms of several hundred sites and domains. These groups sell bulk backlinks with any anchor text and for a short time at least, the site goes up in ranking. Usually, Google figures it out and slaps penalties on the farms. Recently a new tactic that sites use is to link to a site with an unrelated anchor text. Within days the site drops several pages as Google sees it as some form of abuse. However, this is easily undone by going to the Search Console and disavowing links from all those domains.
This is the worst type of negative SEO. Sites scrape content from your website and post it on theirs. Usually Google is smart enough to figure out which one is the original. However, there are times when Google indexes the copy first and then the original. Use a tool like Copyscape to identify scraped articles and then use the Google copyright infringement report.