Do not block sites
Do not block a site directly when it goes out of limits. That is irresponsible. Instead, let the user select what should happen when a site goes out or limits. I want my site to auto-upgrade and if there is a problem that caused it to go out of limits, I can correct it and downgrade later. Other users that has less critical sites (are there any?) can choose to have the site blocked instead. Most fair would be to send out a warning though, and block after 12 hours or something. I hope you listen.
Thanks Christer for the suggestion. This is being worked on. We are planning on offering a feature where you can allow us to auto scale your site for you based on limits that you set such as a budget and length of time. More details to come...