IOS – iPhone App to limit the data usage to 10 Gt

I know this doesn't answer the question specifically, but are you sure you are on wi-fi when its available and not just solely on 3G? I've made that switch in the past and forgotten to go back. Also, have you installed any apps recently around the time the spike occrured?

No, that is not possible: it would mean Apple allows developers access to deactivating data access. I guess you can understand the security problem that would mean: an app could decide on its own you don't have data access anymore.

The only way to do that is to regularly check your data usage in “Settings > General > Usage > Cell network usage” (menu names may not be the exact ones, my iPhone isn't in English, sorry). You can reinitialize these statistics each time your limit is reset.

If your limit is crossed, switch data off in “Settings > Network” (iOS 5 only).