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 in the top right corner. Then, click More from G Suite Marketplace at the bottom.
The G Suite Marketplace page opens. Click the gear icon > Manage Apps.
Click the three dots > Uninstall.
Or click Uninstall directly from YAMM add-on.
A pop-up appears. Click Uninstall app.