capping my internet usage

[#1]

hi
I have just set up broadband today, but when I joined I was told that I can cap my usage,so I dont go over my allowance, as I'm on the cheap rate with limited date. how do I set this up, so I dont have to pay anymore than my contract allowance.

[#2]

Go to here and set the Maximum Extra Monthly Spend to zero
https://portal.plus.net/data_transfer_watch/