How can I make my site stop adding  tags in textareas?
Sounds like maybe you need to disable the "Convert line breaks into HTML (i.e. &lt;br&gt; and &lt;p&gt;)" filter on your Full HTML text format.

This can be done by navigating to Configuration > Content authoring > Text formats and editors, then editing the Full HTML configuration and un-checking the box for "Convert line breaks into HTML (i.e. &lt;br&gt; and &lt;p&gt;)" under "Enabled filters". Assuming this is Drupal 8 anyway. Drupal 7 has the same setting but the exact location might be different.