I've had some middling success with this by utilizing CLAUDE.md and language features. Two approaches in C#: 1) use partial classes and create a 'rule' in CLAUDE.md to never touch named files, e.g. User.cs (edits allowed) User.Protected.cs (not allowed by convention) and 2) a no-AI-allowed attribute, e.g. [DontModifyThisClassOrAttributeOrMethodOrWhatever] and instructions to never modify said target. Can be much more granular and Claude Code seems to respect it.