But I need to prevent this behaviour moving forwards. Suffice to say she gets a varied diet, and has cuttle fish in her pen, plus I apply calcium vitamin supplements to her food, so she is getting enough calcium. But she has developed a penchant for seeking out dirty bits of natural chalk and trying to eat them - and indeed, other small bits of stone. She does this when I'm giving her a wider walk in the garden - weaving in and out of bushes and mooching along the soil. She just seems to explore everything by putting it in her mouth

I'm re-doing her outdoor enclosure this weekend with entirely new top soil and play sand mix to avoid ANY bits of chalk or stone being in her designated area. But she is adventurous and easily bored, so I try to give her at least 1-2 opportunities to walk around the wider garden a day (especially in the summer). I always supervise this for fear of the usual dangers, and have ensured it's all safe in terms of plants. However, what I can't realistically do is remove every tiny bit of chalk (or indeed little bit of rock that she thinks looks interesting).
So, do you have any advice on how to deter the behaviour? Currently standing over her and saying 'no' is getting me so far, but isn't viable in the long term. And she adores having the ability to walk around the wider garden, so I don't want to stop her having that fun. NB: I had thought "is she actually getting enough calcium, and is that the cause?" but the vet has confirmed that bone density etc are all good, so we don't think it's a lack of calcium that's driving it. More incessant curiosity!
Would welcome any advice you can offer. Thanks