this is not uncommon behavior in cats
spaying might help with the behavior, but at this point not likely. it has become a learned and normal behavior for her.
you need to eliminate her access to bags, rugs, and clothes piles and encourage use of her litter box. (she might not like the kind of litter you use, and you also need to make sure it stays clean, some cats want their litter box cleaned as much as three times a day! But at the minimum it should be cleaned at least once/day.
there is also a product called Feliway that can help with problem marking and other undesirable traits:
http://www.feliway.com/usI would like to know why the vet claims she cannot be spayed, that is unusual and would imply that perhaps she has a major health problem, which in turn could be lending to her behavior.