4 things you can do.
Firstly join the forums so you can post replies to this thread and share ideas, thoughts and help with others on other threads
As for Snapchat:
1 - depending on her age it might be worth simply talking to her and explaining the dangers / your concerns? My 9yo surprisingly responds to this but it's not for everyone
2 - put restrictions on the phone and remove the App Store so she has no way of downloading it
3 - setup family sharing and turn on Ask to buy then remove snapchat from purchase history so it can't be downloaded without you know I got BUT let's her download other apps
Can I ask how old she is and why you don't want her to have snapchat?
I let mine have it but she's only allowed to have family on it (which I check).