My internet provider, Mediacom, sent me a letter saying they they are tracking my downloads. How do I put them on the block list? How do I get their IP address to add them?

You can't really since if you block their IP then you'll lose internet connection. Plus, they can monitor your traffic regardless what IP you block. Check the contract/agreement you agreed to when signing up for service with MediaCom.

A Secure VPN connection would encrypt all traffic and they wouldn't be able to see what your doing.

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This business is binary. You're either a one or a zero.

To answer to your questions to some degree on how to create your own custom blocklist:
1) Create a text file (.txt).
2) Add line entries to text file of yours in format of [range_description]:[range_start]-[range_end]. An example would be: "MyBlockCompany:100.101.102.103-100.101.102.104".
3) Change it into .p2p format via renaming and afterwards add this file to PeerBlock via List Manager.

I'm afraid I won't be able to know a way to get all of their IP addresses, but I would recommend that you do what zerocoolxp suggested. Also if needed, change your ISP.

a bit OT I know but if my isp did that I'd drop them like a sack of potatoes. There are plenty more isp's out there and you don't have to take crap from anyone. Find another isp and migrate away from mediacom. Cancelling your account shows them you mean business and when they beg you to stay - which they will - teach them a lesson, stick to your guns and dump them.

I really, really want to, but I don't want dial-up and don't care for satellite. Mediacom is the only cable company in town. However, I am going to be moving to another town soon that Mediacom does not control. I hope Cox doesn't do this too.