Maybe the biggest challenge is defining what constitutes "spam." While some cases seem clear-cut (e.g., repeated identical messages from bots, malware, phishing), others are quite subjective. Subtle marketing? Aggressive marketing? Repetitive but sincere advocacy for a cause? Repetitive but insincere trolling? Repetitive but sincere trolling?
All this seems rather obvious, so I was kind of surprised to see how many people bought into Elon's vision for Twitter, it was never workable.