"BackService" Not Opened pop-up won't go away

BackService is a type of adware targeting Mac users.

You can try Etrecheck which may help to remove a Adware

Or you can try https://www.malwarebytes.com/

There is Adware & Malware which will affect the computer and is generally downloaded from “ Shady “ Websites or Developers who Inject this software into the download.

From a well written User Tip from @Kurt Lang

What is malware? - Apple Community

Add on

Thanks to a long standing contributor @John Galt  

Stop unwanted Notifications

Phony "tech support" / "ransomware" popups and web pages