Depending on your device, you can set it so that it alerts you at your data limit and you can then disable data usage. There are likely 3rd party apps to do this as well.
All carriers will block your data overage @ $50 under the Wireless Code of Conduct.
Hi, tsizzle is correct and I would also suggest a data tracking application. You can set your limit in a specific 30 days time frame and avoid extra fees. You can also access your telus mobility online account, from the app or the website to see your usage. Which device are you using? We'll need to know in order to help you more, thanks!