Kerrie ( @330467 ), quota numbers like this frequently are the result of media files and attachments to conversations. For example, if a student takes a picture with their cell phone and then send it to their teacher those can quickly fill up the 50 MB. If someone records a media file, those used to count against the quota as well. I've seen faculty with huge numbers because they left video messages to their students. You see, people had to upload the file and it went into their personal space and then they could use it within Canvas. Attaching images to discussions used to count against the quota as well. I vaguely remember some kind of adjustment within the last year or two, but I'm not positive on that.
It is unlikely to be a bug in calculation of the quota, but a misunderstanding of what counts against the quota. Once the quota is exceeded, some files may still be uploaded and others may not. Students not being able to update their profile picture is a sign of being over quota.
I would go to the Account > Files for the person who is over quota. Then look at My Files > unfiled. This is, in my experience, the most likely place to find excessive files. I sort by the size so the largest files come to the top. In addition to the ctrl/cmd click that was mentioned, you can also click on the first file to delete, then hold the shift and click on the last file to delete and it will select all the ones in-between. You can also use ctrl-A for select all and then deselect some.
There are also a lot of folders named for courses. Files in those do not count against the quota.
Somewhere in the Community, there have been some extensive discussions about what counts towards a quota. Unfortunately, I have to leave in 10 minutes and don't have time to research that right now.