I get it now - its a freewheel, not a cassette so you need to unscrew the whole thing.
Normal process is to use a freewheel removal tool into the freewheel and unscrew it using the rim as a lever,
In your case the freewheel pawls are not gripping when driving, so the rim will turn freely in both directions, and not unscrew.
- Blast the middle of the freewheel with penetrating oil, to try and get a pawl to fall into place enough to undo the freewheel.
- tap it gently with a small hammer, again to get a pawl to drop
- bash it firmly with a medium hammer. Don't worry about saving the freewheel at this point, denting it is okay. Remove the axle and all the bearings. don't want to brinnel/damage your bearing races.
- wail on it with a large hammer.
- Last resort is to mound the wheel flat with the freewheel facing up, and cut through the freewheel with a thin cutting wheel in a 100-125mm (4" or 5") grinder. I've done this and even a cheap knockoff freewheel was hard to remove.
Your goal is to get a good slot in the body of the freewheel, but not to cut the threadded part of the hub at all. The pesky cogs are just in your way, you can cut them off.
I've done the grinder thing, and it was on a 20" wheel where I just wanted to save the spokes. So I wasn't careful to avoid the hub, and this made it harder to undo because of the slot I'd cut, and that it was cheap nasty pressed metal.