Here's a small guide to the best way to deal with someone using your content in a way that violates your terms of use. This is only a recommendation, there is not really a "best" way of doing it. But I think it's a very constructive way of doing it, anyhow.

This guide is mainly intended for people who use the NMG Framework, but even if you don't, this might be helpful.

Now, first of all. Most people in the world are decent people, like you and I. So it's a good idea to actually make sure this person is being outright negligent, or if it's just a simple misunderstanding. Look at the mod page. How well written is it? Perhaps the individual in question is younger, or an ESL, and haven't fully understood the terms of use.

These reasons makes it a good idea to try and establish a dialogue. To send them a PM explaining the situation and how to fix it. Or even if you feel like they've done a good job with the mod and it's obviously a misunderstanding, and it's not really causing you any problems; Why not just grant them permission? Everyone makes mistakes, after all. I understand if you don't want to. That's perfectly fine. But that's always an option as well.

Then, wait for a response. The response will really determine the next course of action. If they then proceed to either adapt the mod to your conditions for using assets, or provide some kind of resolution. Then problem solved, right? If they refuse, or act hostile, contact a moderator.

But it's always good to try the diplomatic approach for several reasons:

1. It provides more mods for the community. It's always better if they remove or replace your assets if you don't want them in there, and keep the mod up for people to enjoy. Than to have it deleted forever.

2. All disputes we can solve ourselves through conversation and mutual understanding, are disputes that will save time for moderators. 

3. These kinds of resolutions typically create a more friendly community atmosphere, and you might even find yourself knowing a new modder who can help you in the future just like how you helped them.

But like I said, if they wilfully violate your rights with disregard to terms of use, just tell a moderator. All I am proposing is to make sure that's actually the situation at hand.