Well I was on Behemoth back when it was pretty much the most populated server, it took nearly a year to fill my blacklist.
On Odin I currently have about 15 people blacklisted in the last 6 months.
If you don't blacklist them, obviously they will continue to message you.

I don't even remember the last time I actually got a tell from one, it's the 1 time a day friend request they try for me. thats it.

If your server is lower population, I don't know about malboro's numbers, it could be due to them going through the lower amount of names quicker.