If the user has a quota set in the panel, to equal or less than your plan limit, and everything is working correctly, then the user should be blocked from adding more when the limit is reached. I've hit that before. However, currently the panel reports all zeroes for disk space usage for all my users, so everything is not always working correctly. I think if you've done your part to set limits, and DreamHost's system failed, they should not charge you extra, but that and a click on my PayMe link will buy us a small cup of coffee.
They need more support help.
Pay me (and DreamHost gets 5%+$0.30) Cut the code: [color=#00CC00][b]9999=$99.99 Off[/b][/color]