Direct Answer: A healthy boundary regarding a partner's ex focuses on your terms of participation in the relationship. You cannot force your partner to stop talking to their ex, but you can define what level of transparency you require and decide whether you will remain in a relationship where exes play a highly active, un-negotiated role.
An ex represents a shared history and intimacy that predates your relationship. When your partner maintains highly active contact, it is natural to feel threatened. However, expressing this threat through anger, demands, or snooping on their phone usually drives your partner into a defensive posture. The key is to separate what you actually observe from what you worry is happening, and map your choices clearly.