I’m a great believer in not making threats you cannot carry out. As a working single mum of three teenagers, I had to think carefully before indisciplined my children. A threat of no internet until I came home from work meant me disconnecting the router every day to take it to work with me - Imwish I had thought of changing the password! A ban on using the skateboard resulted in the skateboard joining the router in my car boot! I kept all the treat type food in my car as well as otherwise one son would eat it all as soon as I bought it.
Not knowing the age of your grandchild makes it difficult to suggest solutions but certainly disconnecting the internet when she refuses to stop would work. For a young child, any discipline needs to be immediate, reporting incidents to parents at the end of a visit will have little impact.
You could offer rewards for good behaviour, age appropriate, so she gets to appreciate the value of doing what you want her to do. Small things like a token towards a treat when out shopping, or towards being allowed on a device later in the day. It’s not bribery, just teaching that good behaviour gets more attention than bad. I know that children should not be rewarded for doing the right thing but once something has become an issue, for a short time positive reinforcement might work.