They don't always come back on monday and clean it up. Sometimes they just let it sit for days.
And even if they did clean up things every monday, I think Rick Steves' company has the resources to deal more effectively with this - they just need to care. Seems pretty obvious they don't. That's their choice, but it doesn't reflect well on them. You know, they're not some little mom-and-pop shop struggling to barely make ends meet; they're a well-established, very successful business with plenty of financial wherewithal.
The fix is really easy: if they cared, they would have someone working the forums 24/7/365 (like most companies that have an online presence do), not just an occasional glance during weekday business hours.
The spammers obviously know that from Friday afternoon through at least monday (often much longer), there's no curation at all, and nobody minding the store. In this day and age, it shouldn't come as any surprise that this lack of attention is being exploited.
The problem will continue to get worse until the company decides to put a stop to it.