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.
You can make a request ('I would feel much safer if you blocked them'). However, because they control their actions, they have the right to decline. You then must decide if their choice to maintain contact with the ex is compatible with your vision of a safe relationship.
Healthy co-parenting focuses on the child's schedule, health, and schooling. When the conversation drifts to marital history, complaints about current partners, or emotional advice, it is moving into poor boundary territory.