First of all, this clearly refers to "Adult-related sites", and that should be a pretty easy thing to determine. Is it, or is it not?
Obviously DreamHost would only be able to terminate your DreamHost account. They have no influence on PayPal. That addresses your concern about "which account".
Concerning "very vague", let's start by pointing out that the DH folks are pretty sensible. I can't imagine that they'll close your account if a forum member posts objectionable material and you don't get a decent chance of removing it. In contrast, if you purposefully post objectionable material for others to access, that would be a violation.
Of course this is just my guess at how DH would interpret this. But as I said, they're reasonable, so surely you'll get a chance to defend yourself and/or correct the matter - at least if you've shown reasonable interest to follow these guidelines.
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