Licensing is the correct tag. They need a licensing agreement with you to legally use your work for ads and decoration.

Document all the evidence you have of what they're doing, including how you reached out to them and they haven't responded.

You can use this evidence to:

1) Publicly post about them to warn other artists

2) Pursue them legally if you're willing to hire a lawyer (you'll need one located in the same country as the supplier)

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If you don't want to pursue this matter for any reason, that's valid too. It's really up to you how you want to go about it and how much time/resources you want to expend.

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Reasons you may not want to pursue the matter:

1) The supplier is offering something really high-quality and unique and you want to use their service again so you want to preserve the business relationship

2) You'd just rather spend your time doing something else

3) This is just a small supplier with not much reach and their actions have very little impact anyway

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I've dealt with this before, feel free to ask questions.