This is permanent. Really, actually permanent. There is no way to remove this item from this page. I can’t believe this is a real problem.
One of my developers did this under the assumption that we’d be able to undo it, and there’s absolutely no way to fix it. Totally insane. We can’t even repurpose the item. It’s just a red square, stuck in the group store, on a group that has nothing to do with clothing, forever.
Unfortunately, a lot of features on Roblox are not test-friendly, and permanent like you said. This needs to change. I wholeheartedly agree with this. Roblox needs to take the initiative and allow developers the ability to disable all types of content from public view, not just places (which by the way is also not completely hidden from the public).
As the person who uploaded this, I’d also like to note that this is not even a T-Shirt, Shirt, or Pants! This is a Decal! Why are Decals in the Store to begin with?
I used a Decal under the assumption that it would stay hidden and let me test our asset purchasing and ownership API. Instead, the Decal shows up in the Store. I would expect development assets like Decals to never show up in the Store in the first place.
How does one purchase a decal 
You can put promotional art, promotional free models/ map kits, and whatever in the store section. It should be fixed to allow us to choose what shows at the front store page though (if at all) but hopefully this comes with whenever the massive group overhaul eventually comes out.
You can hide the item from the store by taking it offsale, although it may take a while to remove itself.
I figured that taking it off sale wouldn’t work at all since it was still in the store and said “Not for sale” shortly after.
I took it off sale yesterday and it’s gone from the store right now. Thanks for the response!