Some Thomson freeview boxes do need a valid viewing card for full functionality. I.e. Thomson DTR6300, DTI6201-25, DTI6021-50.
The manual for the Thomson freeview box will may explain this and more.
Some of the Thomson TUTV boxes will not work without a valid viewing card. This is due to their orignal subsidised package deal that they were sold under. If this is the case for you then you will need to contact TUTV with the model and its serial number and persuade them to issue an over the air unlock for your box.
A long time ago it has also been reported that inserting someone else's TUTV viewing card from a subsidised package also can lock the freeview box.
Once the box is unlocked you should delete all the TUTV channels. These are not be viewable without a card and so you will not be missimg anything. What deleting the TUTV channels does is to release a chunk of the disk space so that you can record more programmes.
NS, December 2012