The T & Cs are fairly clear, under the Usage section it reads -

If you exceed your Usage Cap, we will contact you by email to let you know that you have done so.
If your usage continues to exceed your Usage Cap, we will continue to contact you by email to ask you to reduce your usage immediately. If after a reasonable period of time your usage still exceeds your Usage Cap, then we may do one or more of the following things by notice in writing:
Then follows the list of what may be done, ranging from charging for the excess amount, to exclusion.