I want to delete old data in my blog

I once used piwik in our blog. A friend inserted a code to track feed readers. Since then I get thousands and thousands of double contend messages in google search console.
Piwik or matomo is long since deactivated and deleted. BUT the code still get’s produced. Our blog isn’t working because of that. Can you please help me to stop this?!
One of those pages looks like that:
I really tried everything and don’t even have an idea where to delete the code that produces pages like that.
Hopefully somebody has an idea? thanks so much,

Hi @HappyBlogger
What do you mean by:

Can you share a screenshot in order I understand this?

Also you wrote:

If Matomo has been deactivated (removed from measurables?), then you don’t need to delete data, they are already deleted from Matomo…?