The TagMeNot tag is free. The original QR code design is released with a Creative Commons license (CC-BY) license, so you can use it as you like, as long as attribution to TagMeNot.info is preserved .
You can download the tag and print your own t-shirts, badges, buttons or have it stitched. Use it wherever you want. You can even sell your own TagMeNot tagged objects. In this case let me know if you use online stores for distribution, so we can list them on this website.
You can design other QR codes pointing to this site, or modify the original one
You may know that QR codes can be modified to some extent: you can add colors, or embed other images. Consider that doing so may reduce the readability of the tag and therefore its functionality.