When I look on the main Eurostar website about tickets, it only mentions specific-train tickets (of varying levels of flexibility), as a single or a return. I can't see anything there about any sort of unlimited use / season ticket type options.
However, a couple of nights ago I was chatting with some people in a bar in Lille, and they seemed to think that it was possible to buy a special ticket (aimed at commuters) for unlimited travel on the Eurostar starting in Lille. They were all sure that you could get such a ticket between Lille and Brussels, and fairly sure that there was a Lille to London one too.
Try as I might though, I can't find anything about these tickets by googling. Do they exist? Or were the locals describing "something that really ought to exist, but sadly doesn't"?