I have questions here, a lot of lobbying is done by:

a) trade organizations (we're all the onion farmers in Nebraska and want to make sure the Nebraska legislature doesn't pass laws that negatively impact us and promote laws that help us)

and

b) activist organizations (we're a coalition of organizations that protect water usage in the Mississippi delta and want to pass laws that promote conservation in those states)

Those groups often choose to retain professional lobbyists but will also send groups of interested parties to lobby who are not professional lobbyists.

Do you also ban trade organizations and activist organizations in this case? Do you carve out exceptions for them and just ban the "freelance" lobbyists? Most lobbying is meeting with legislators and talking with them about issues, educating them. How do you ban that without making legislators effectively useless (or if you're cynical, even more useless)?

This seems like an easy proposal, but I don't see many case studies of it being successfully implemented.

I can't really think of any "wholesale all lobbying forbidden", but at least for specific industries there are a couple. "WHO FCTC Article 5.3" which is about limiting the influence of tobacco companies is probably the first that comes to mind, and the most famous example. Singapore I think recently done some legislative changes around lobbying as well, but I'd confess to not knowing much about it, maybe someone here could fill out the blanks if they have the knowledge already.

Running a government and banning the representatives of your economy from talking to you is insanely stupid.

There is absolutely nothing wrong with lobbying, it is an essential part of the government and can not be legislated away, without crippling the entire country.

Not possible in a country with freedom of speech protections

Maybe not possible in the specific country you're thinking about, with their specific implementation of "freedom of speech", but it's hardly the only one, and not all of them are incompatible with outlawing lobbying (if the US one even is, I dunno).

If you try to ban lobbying you will incidentally criminalize basically all political speech. White papers are a form of lobbying, providing testimony is a form of lobbying, running an ad campaign is lobbying, speaking with your congressional representative is lobbying.

It'd simply have to be carefully drafted policy rather than an internet comment.

So simple it's indescribable

This notion always puzzles me. It's not a complex idea.

Just like, say, banning GMO bananas. But such regulation is a whole text which may need to define or refer to definitions of "GMO" and "banana", specify what's banned, exemptions, enforcement authority, penalties, and so on. Maybe 10 pages of legalese. It requires time, expertise, research. But it's still just a ban on GMO bananas.

Or a programmed UI button to show a message. Simple. The specifics of the execution are a separate matter.

It's not "indescribable", but no one will describe it to you ad hoc and expecting it is silly.

What's "it"? Speaking in public?

Making strawmen. Your position of "policy is impossible to write" is inane.