One thing that is quite clear from the discussion is that proper guidelines need to be in place under any future system as to what constitutes spam. I'm of the belief that it won't be possible to make decisions on spam entirely objective so would suggest something along the lines of guidelines for what definitely constitutes spam and then anything else is down to the judgement of the trusted few.
Simple first guidelines could be things such as:
any post that is under x characters/words
any post that is incoherent or even in the wrong language
I can't think of much more immediately right now but I'm sure if everyone puts their brain power together a short list of things that are 100% spam or close enough can be agreed upon. This list can then work as a starting point, it can be written up for new members to read so that they are aware of it and then it can be used as a basis to at least remove a level of the spam. The more complex spam is going to need dedicated people and the right people to ensure it's tackled effectively and fairly.
Length should not really be a factor imo. Sometimes a simple 3-5 word response is all that is needed. The issue here is that you can not just make a set rule of what is spam and what is not...everything is subjective. Say someone asks a question and they get 20 answers from 20 people that all basically say the same thing. Obviously people are replying for the posts even if they are all worded well...
Length within reason should not be a factor but I think it would be safe to say for starters that any post that is 1 word only is definitely spam. The case you described isn't really something that can ever be covered by any set rules and that's where you would need people to be able to make judgement calls.
But if you could get to the stage where you had something that said along the lines of 'the following can and will be considered as spam, however this does not mean that anything that passes these guidelines can also not be considered as spam'. It's at least a starting point for the community to have some clarity as to what is expected or what can be punished.