Go to the police.
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What proof do you have he is doing this? It may not be legal (many jurisdictions have not kept up with technology though) but a court will not entertain conjecture, hearsay, "hunches" or speculation. You need admissible evidence. If you have this evidence, you can also call the non-emergency number of your police department and see if they want to get involved (only the government can prosecute crimes). Cops usually do not like being involved in personal disputes such as this though, but you can see. You can also talk to your cell phone company and see if they can be of any assistance. You can also have a local lawyer send a cease and desist letter--that might get his attention. However, most lawyers want to know there is some basis for the letter--so you're back to your evidentiary issue again.
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