Unable to lower broadband useage

[#1]

I'm on an old teleworker account which is supposed to have a £16.33 + VAT basic cost per month.  I seem to be regularly charged for a 10 gig transfer limit and over £20 + VAT per month, which I cannot reduce because I see nowhere to be able to do this on my useage management page.  I've not used more than 7 gigs for the past 3 months in any case. (cannot see further back).  Where can I reduce my useage, please ?  I am only given the option to increase.
Thanks in advance.

[#2]

Usually you should be able to set additional usage to zero, however this won't work if the limit has already been exceeded. For example if the usage allocation was breached and 5gb was added, setting it to £5 would ensure nothing above that block of data would be added.