No, you can't cancel it.
I used this for a 5* hotel in Edinburgh: We ended up in the Caledonian Hilton with an awful old room, really small,with no bath, with a manky carpet, in the older part of the hotel. It was not 5* standard at all.
I complained, but in order to move to a better room we were charged quite a bit, with no option of just being moved to a new room.
I wouldn't use Secret Hotels again, on the basis of that. Other people may have had better experiences.
I felt that the Secret Room deal in that hotel was just to get rid a room that wasn't up to scratch.
Nicola
Ps I complained to the hotel in writing, and got a standard letter back saying 'thank you for your helpful comments', and not addressing my complaint at all.