Deactivating or Deleting Plugin Desroys My Website

Hello,
I have a plugin installed and activated, but I am not using the plugin. Once I deactivate or delete this error code appears on all pages and posts:

Fatal error: Call to undefined function headerads() in /home3/ournewpr/public_html/withlacoocheegulfpreserve/wp-content/themes/hmtpro5/header.php on line 754

I would like to get rid of this plugin without hurting my site.
Please help! Thanks.
Jeff
The problem is on this website: https://withlacoocheegulfpreserve.com