Maybe a solution should be that you can reply to PM's with the starting level rep (so lost rep removes that privilege), but you can only start a pm to someone new with 500 rep.
I think the point of the system is that a spambot would need to make many posts (more likely to get caught) before they can then spam everyone's PM's (it's easier to delete one thread than a tonne of PM's)
ut are they really going to take the time to comment or add a like on peoples games?
Yes, they definitely do sadly.