This article explains how to remove Yet Another Mail Merge from your Google account.
If you want to remove the add-on from your spreadsheet, you will have to remove the add-on from the G Suite Marketplace.
From your Google account, click on the menu at the top left side. Then, click ‘More from G Suite Marketplace’:
The G Suite Marketplace page opens. You can either click on ‘Manage Apps’ > ‘Remove’:
Or click ‘Uninstall’ directly from YAMM add-on:
A pop-up appears. Click 'Uninstall app':
That’s it! If you check on a Google Spreadsheet you will see that YAMM has been correctly uninstalled: