Hey Natty,
Are you sure that it's not just a passing phase? Have you tried to firmly say 'no' to her? I'm assuming you probably have done all of the rest & I'm not trying to persuade your decision.
I had a hard time getting Jessica onto formula to begin with, she was 8 weeks old when I put her onto formula & I just had to keep on perservering with it. In the end, my mum came for the day & she'd just try & try over & over to get her to take the bottle. Jessica had a fascination with fans, so in the end I would gently push the blades of the fan so they were going around & interesting Jessica & my mum shoved the bottle in her mouth & she took in 30mL without even realising it & it became easier from there.
HTH!
Love Jayne