How would I go about getting my cats to stop using the litterbox?

When I got them I lived in the “big city” and they spent the first year never going outside. Now they have a acre to play on and spend 75% of the time outside.

But they still refuse to shit outside. They beg to come in and then poop in the box and beg to go back out.

I tried removing the litterbox and they just started pooping on my clothes. It seems ridiculous to pay for litter and clean it everyday when they should just be going outside.

I’m not thrilled with the idea of just not letting them inside until they get used to doing their thing outside but I am running out of options.