Hello @Orthodoxuk . Do you mean block individual people on her account? If so, directions are here: https://help.snapchat.com/hc/en-gb/articles/7012401093396-How-to-Block-a-Friend-on-Snapchat
If you mean how to block the app totally, then it would need to be uninstalled from her phone. however, I'm not sure what will stop her reinstalling it unless you have sole control of the phone's identity. (What I mean is, for an iPhone, as an example, it would need an Apple ID and you may have the detail and not give it to her)
