I have been getting these for quite a while now and honestly I think we simply can't. Perhaps an option could be implemented to block messages from people with the newbie rank. I don't want to disable my PMs.
Unfortunately we can't block particular rank. To be honest even if I could, I wouldn't
At least one ICO offer a day knocks at my PM door and among those, some real "gems" can be found.
The day before yesterday I was hit with an offer written with so ridiculously broken language that I almost spilled my tea.
There was something about furniture loving [sic] and jeans... man, bot translators are still in alpha phase.
One or two PM's a day, with such "lifetime" deals are not that terrible. I can accept this status quo.
There is a price for having active, high ranked account