It doesn't work, no matter what I press, and wherever I click, I can't set a filter or block a strip.

I'm not even sure where to begin troubleshooting.
It doesn't work on a fresh save (command "/cheat all" to quickly get some splitters)? Do you have Alt-mode enabled to see the deconstruction planner filter appearing? Does the button on the side exist on when splitter UI is open? Is the mod enabled in mod manager?
Could I get a save file where it doesn't work?

In order:
- I haven't tried a new save.

• Yes, alt mode is always on.

• Yes, there is a button on the side of the manipulator window.

• Yes, the mod is on, otherwise there would be no button (most likely).

• No problem, available at the link: https://drive.google.com/file/d/1w0tbKwaYCPOv3cHrO9Yp0UV56eehEWpJ/view?usp=sharing

Okay, so it's not working on the mainbus splitter because both output belts are connected. There's nothing to "block".
It blocks only when a splitter has one output and has no priorities set.

Look at the information page how the splitters have only one connected output belt.

Hmm....
Hmm....
Most likely I misunderstood the mod description...
I don't quite understand then, why block the unused divider strip if nothing will go there anyway?

Is it possible to add functionality where the user will be able to block the band being used?

And thanks for the clarification, quick response and time!

It's so you don't get dangling items on ends of a splitter. Yeah, it's mostly cosmetic.

Inversely, I'm not sure of the use case to block a belt lane on a mainbus.

It may not be purely cosmetic any longer. What happens if you've got eggs on the belt and don't block the unused side? I can guess what the effect would be for having those 4 little, unimportant items, stranded on the unused belt.