Setting boundaries can be difficult. If another employee isn’t respecting your work boundaries, there are several ways you can address the problem. Depending on your relationship with the employee in question and the severity of the transgression, the first step may be to negotiate your boundaries directly. If you feel safe, start by making your boundaries clear and requesting a behavior change.
If this doesn’t feel like a viable option—either because of an unhealthy power dynamic or because of the specific behavior—the next step is to raise your concerns to management or human resources. It may be beneficial to keep records of any evidence of the behavior in question, such as copies of inappropriate emails or a list documenting each incident.