You can delete unused UET tags that aren't associated with conversion goals, audiences or shared with other accounts. Once a UET tag is deleted, it will no longer show in the grid. Don't know what UET is? See What is UET and how can it help me?
From the top menu, select Tools > UET tag.
If you're using the new Microsoft Advertising navigation, from the navigation menu on the left, hover over Conversions and select UET tag.
If a UET tag doesn't meet the requirements for deletion, you will see an error with the number of changes that weren't made. Select Show errors to see which conditions weren't met.