Didn't attack the substance of my argument. "That leads into an absurd kind of "can we really know anything" regression." is basically you attaching a negative attribute to what I said.
I don't believe you read your own definition closely enough. You defined ad homs as arguments which avoided the topic at hand "by instead attacking the character, motive, or other attribute of the person making the argument." Since I have at no point attacked the person making the argument (namely, yourself), I have not committed an ad hom. As I said earlier (and as your own definition verifies) ad homs deal with attacking people, not content.
Everything is about the foundation
I am not unwilling to discuss the issue with you up to a point. But you've gotten here to the crux of why I have reached the point where I am unwilling to continue the debate. If every debate has to drill down to some sort of intellectual "foundation," then the actual question which sparked the debate will never be discussed, because the conversation becomes bogged down in a never-ending series of prior questions.
This has a lot to do with the stasis theory of argument. If we are constantly having to sort out questions at lower stasis levels, we shortchange the higher-level issues. I am not interested, therefore, in debating whether we can determine intent, because it strikes most people as obvious that we can. Having to debate these kinds of commonsense issues is therefore counterproductive and regressive; if we actually want to talk about spam, then let's talk about spam. Let's not talk about whether intent is determinable.
Generally, most posts have some purpose, because humans are purposive beings. Whether that purpose is communicative or not is what is really at issue, not whether there is purpose. Content has communicative purpose if the content itself (that is, the words themselves) are designed to communicate something. Someone who posts "f" once in this thread can be reasonably inferred to be doing so in order to communicate support to RM. That "f" then has a communicative purpose. Someone who posts "f" 70 times in this thread can be reasonably inferred to be doing so in order to boost their post count, disrupt the thread, etc. Clearly, 70 identical posts aren't serving a communicative purpose in and of themselves, even if they may be communicating something performatively. In such a case, it is not the content itself which is communicating, but the volume of the content which is. That content lacks communicative purpose as I explained it. Where communicative purpose is in doubt, moderation errs on the side that it is present.