Can someone build Spammy Backlinks and hurt my Search Rankings?
YES! This is absolutely possible. Negative SEO is a reality which cannot be ignored and is used by people, usually, who cannot afford SEO for their own Websites as their Websites lack original content and are more or less likely not to be able to achieve a position on 1st page of Google.
There are instances where people employ SEO Experts / Agencies to use Negative SEO to bring down a competitor's Website's rankings in SERP by building Spammy Links to a competitors Website .