Block On Social
Make sure to block this person on all of your social media platforms so that they don’t have any access to your personal information and no way to contact you. This can help reduce the harassment as well. However, this doesn’t prevent them from posting publicly, just directly contacting you.
Contact The Authorities
If you feel it is necessary, perhaps involve the local police and file a report. Unless you’re in direct danger, there’s no need to call emergency services. Give them all of the details you can and make sure you provide the evidence.