Integrated shared calender cannot be removed

Hello, I opened a shared calendar I downloaded from a link sent to me by a colleague. Stupidly I clicked integrate on opening, and now it sits within my own calendar, and I can't remove it. It doesn't come up on the left side in the pane either. I would like to get rid of this integration.

As part of problem-solving, I made the calendar go to default and now on top of the above issue, I can see all the details of my calendar invites (so the notes details) as well as the normal subject line. I can't work out how to remove that as well.

Please help me, thanks.