Data just stops when you use it up so it automatically caps itself. If you can't trust yourself with calls and texts get a tariff with unlimited both. Download the My O2 app to keep track of your usage. http://www.o2.co.uk/apps/my-o2
I have no affiliation whatsoever with O2 or any subsidiary companies. Comments posted are entirely of my own opinion. This is not Customer Service so we are unable to help with account specific issues.