Ben, my version is 10.3.1, but I am pretty sure it's the same in 10.2...
Right click on the dataset in Catalog. Scoot down to the Spatial Reference part, click edit to bring up the spatial reference properties page.
Click the little pull-down next to the world icon at the top. Click Clear.
And, in the case of external files like tiff or jpeg, you may have to use the normal win file explorer to delete any associated files like worldfiles - tfw, and any aux or xml.