Hopefully you've figured out something with your problem, but I thought I'd clarify a few things. The IMEI blocking has always been a very common thing for years with Verizon and Sprint, and very commonly related to unpaid bills. T-Mobile and At&t have always had the capability as well, but have only recently been enforcing it more.
The HTC One S (specifically for T-Mobile) is compatible with At&t 3G, as well as all GSM carriers in Canada, as this is one of the few T-Mobile phones that has the 850mhz and 1900mhz bands that are compatible with them. The phone also has the standard 1700/2100mzh bands that T-Mobile has always used. This version of the phone does not have the 900mhz, which is commonly used in Europe and Asia.
You can get the phone unlocked from the T-Mobile network for $5-10 by searching on places like eBay.
I don't know what T-Mobile's policy is on IMEI blocking, but it would seem like they would be more lenient on unblocking the phone once they have sold off the unpaid balance to a creditor.
While I do understand your frustration on this, you have to realize that the carriers do this because they take a loss on phones by selling them to the customer at a cost that is way below what the carrier paid for them, with the intentions of recouping the loss over the course of the contract. If I had my phone stolen, I would definitely want them to block the IMEI, which they can do. If I thought I could still use something that I hadn't fully paid for, I would expect to be blocked for that too.