The U.S. Congress example is not very appropriate, since the NNS should be infinitely designed like the Internet Computer, i.e., infinitely scalable. The blockchain world is a digital world, and thus many concerns in the real world do not matter anymore; as for now, I don’t see any real problems of absolute freedom of speech in the NNS.
I think that in the end every proposal should be just a piece of code so that every proposal can be implemented automatically as soon as it is adopted. That is, everything should be precisely defined so that it can be realized or implemented in code.
Guidelines for submitting good proposals are definitely necessary, but it is better to put those guidelines in the Internet Computer Wiki or some other official websites. The Dfinity or the community leaders should educate people on many things, but should not force people to do anything.
I presented a simple idea to deal with the Spam proposal issue yesterday that you might be interested in:
If a listed followee finds a “Spam” proposal, then she can immediately fold this proposal so that her followers will not automatically see it. If a follower wants to see what she folds, then he can just click some button to see the folded; if he think she always folds proposals maliciously, then he can unfollow her or even submit a proposal to unlist her.
I think this idea can cause minimal distortion of the voting market. It is so simple that someone must have already recommended it elsewhere.