Which items don't pass hoppers?

I want to create a sorting system, but the only ones I can find rely on you having items that do not pass through hoppers. I have tried everything from named blocks to cauldrons. All of them go through.

What items can I use?

That’s not how sorting systems work you get items to stop in hoppers by powering them with a redstone signal

Into empty slots - all, no exceptions.

Into slots that already contain an item - only items stackable with that item.

Also, if the hopper is pointed at a non-container (say, into a wall), it won't output its item - they can only be pulled out by another hopper underneath, providing it's not locked. If all the slots are filled by certain items, only items of the same kind as already present will pass, unless some slots free up.

A sorting system works by reading the hopper fullness level (through a comparator) and unlocking another hopper underneath if that fullness exceeds a certain threshold. If the hopper has all of its slots with some items, it can only accept more items of the same type it already has, that's how items are sorted out of the item stream above it. Pulling these items in causes its fullness to increase and items from the first slot are pulled out (and into storage). Fullness drops, hopper underneath gets locked again.

So, since items from the first slot are pulled out, you must assure it never goes empty (then anything could be pulled in!) - so you make sure your input stream never contains items you use to fill the remaining slots - or just pre-fill the hopper only with items of this specific kind you want it to sort.