How can I stop spam filling my junk folder

Several weeks ago my junk folder on my iMac started getting lots of spam, around 12 a day like my antivirus software is inactive, my cloud storage is full, etc. They are all from no-reply@ xyz.edu (xyz varies) and I block them but they reappear from a different sender. How can I stop this? I'm using Sequoia 15.6.1