This is very common with kids her age. I would suggest that they consider putting her into clothes that are not easily removed. Overalls are a great option, pants that have buttons rather than snaps or elastic waists, etc.
You could certainly give potty training a try, but they may not be old enough to do it successfully, it just depends on the individual child. I'd certainly give it a try though. Have they shown interest in the potty? Some little girls are ready to potty train at 20 months, but many are not. It wouldn't hurt to give it a shot though and see how they do.
Unfortunately, beyond that the only thing you can do is make it as difficult as possible to get the diapers off, and discourage them from doing it, but it sounds like you're already doing that. Also, make sure they have lots of opportunities to play with playdough and similar products, perhaps having an acceptable outlet for the urge to play with that texture will help curb their desire to go toward something so undesirable.