Noise unwanted in zoom meeting

I had an unwanted noise for the first time in my zoom class that I could not identify.  Was very annoying.  Could not determine where it was coming from.  But, following the zoom meeting, I received an email that someone had joined my meeting without permission.  Didn't know who it was.  Never saw a person on the zoom.  Didn't recognize the name.  How can I prevent this person from disturbing our class again?  Thank you