It seems that if the fault is still current, you can:
Go to Service Status
Click on the red envelope next to the fault
remove you number
This will then unsubscribe you. Unfortunately, if the fault is resolved (as the one I am getting messages about is), it won't be listed in the Service Status section and therefore you can't unsubscribe.
Apparently Virgin have a 'glitch' at their end at the moment, I have made them aware of it (and pointed them to this thread) and hopefully they will resolve it soon so we can all stop getting spammed!