from a legal perspective it doesn’t matter if you charge for your work or not. Its still intellectual property. That being said, doing something about it is a totally different animal. If you really wanted to pursue it then I think your first course of action would be to submit a cease and desist through an attorney to have the intellectual property removed from unauthorized sites. Actually litigating it would be a tremendous undertaking and require a lot of money.
In short, all it would do is remove exposure of your mod to users who don’t use the site you intended. But that’s your choice as the developer. I think you have every right to decide what happens to your property. But I think taking it as a complement and moving on is the best course of action to reduce your stress on the situation.
I’m sorry people steal things. Unfortunately its just the nature of the beast. If you sold the product to a company with financial resources and the income was worth it to them, they would certainly litigate it.