First, the vet visit, to rule out medical cause (take a stool sample, to
rule out continued roundworm or even a new species of parasite). Then all
I can think of to do is cage her, just to help home her to the litterpan
and to break the ugly cycle she's gotten into. Could she be making a
statement about the litter itself? It might be worthwhile offering a
different litter, if that hasn't been tried already. Either way, though,
I would get her out of her current environment.

Good luck and keep us posted.

Sharon Talbert
Friends of Campus (feral) Cats
www.campuscats.org