Here is a simple and quick solution that I believe meets your requirement.
I assume that all your users have accounts in the /home directory. All you need to do is to change directory to the /home directory, and then do a du at a depth of 1.
cd /home
sudo du -d 1 -hYour output will look something like this:
root@sisrv:/home# sudo du -d 1 -h
395M ./Backups
2.1G ./john
168K ./debian
1.1G ./sam
Or for finding the problem users (directories too),
du -xk | sort -n | tail -25