What box are you referring to that they locked? Cable companies usually don't have a NID (network interface device) on the side of the house, this is only found in phone and fiber. Whatever box they locked is most likely your property, and as such you can either break it open or cut the lock off (as long as you own the building).
What they did basically amounts to vandalism, and I would call the police and file a report (to get the incident on record). They have no right or business cutting wires inside the demarcation line, especially ones that have nothing to do with their service. They are yours, you own them, and you could sue Mediacomm over this, since they caused service issues with not one but TWO other providers.
Take detailed pictures of everything he did before you have it fixed.