I don't think you can stop BYOD and BYOA (Bring Your Own App), you need to work with it.
- Option 1: Try to stop it, but this may become increasingly difficult over time with user-driven IT.
- Option 2: Focus on System of Records (what to control) vs System of Engagement (what to provide guidance on).
- Determine informal vs formal content, e.g. ISO9001, records, etc
- Create a policy for informal content, e.g. Box, Evernote, Dropbox
- Create a policy for formal content, e.g. contracts, agreements.