Funny. I simply put "I'm not interested in recruiters or open positions at this time. I will mark them as spam." at the top of my profile. The spam connection request have stopped completely.
I used to have this in my 'how to contact me" spot in the profile, but LinkedIn failed to surface that section correctly in the public profile. Half of this is their fault.
I have a similar blurb in my profile, although I don't go as far as saying "I'll mark them as spam". But I have a very explicit section, labeled "NOTE TO RECRUITERS I am NOT interested in ..." in there.
With that, I do still get a few recruiter contacts every now and then, but it's just a trickle.
I used to have this in my 'how to contact me" spot in the profile, but LinkedIn failed to surface that section correctly in the public profile. Half of this is their fault.