If it were in physical conversation if someone consistently "spammed" a conversation, by say saying something outloud and obnoxious to get attention, I would ask them to be quiet or leave. If they became persistent the only last resort option I would find would be to ignore them altogether. It might feel morally wrong to simply ignore and pretend another person doesn't exist, but it might be the only thing that does the trick, it might cause them to rethink their words and actions.

That being said I feel like this is way it should be handled online, something like hellbanning being the last option. Unfortunately the difference between real conversation and online is that online a person can simply reenter a board with a completely different name or "face," it could be literally impossible to tell whether a new board spammer is a first-time offender or someone who's been peacefully told to quit in the past. I'm not sure that off-line morals and psychology applies here in the same way