Does anybody else do this? Any thoughts, from the recruiter or from the dev side?
What might be useful is some sort of recruiter CAPTCHA that asks them questions about the position. Recruiters need you to self-qualify because naught but a handful can do that (none if your LI profile is blank). This would automate that.
Keep in mind they might not have something like salary info because the hiring manager doesn't want to set a range for a high-need position.
But at the same time, it would filter out shockingly high percentage of recruiters who ask for "full stack" developers and think that's the whole thing. They literally don't know that there are different stacks. One asked if I could find her any since I run a Java user group. Turns out, she needed Node.js developers. This is common.
So just by asking the primary language and framework, you could filter out the ignorant ones. However, that won't stop them from messaging you in the first place
I'd also mention that learning how to answer questions (or deflect properly) that make you uncomfortable or that you don't want to answer is a good skill to have.
[1] http://www.joelonsoftware.com/articles/fog0000000043.html
What result do you want? They're either going to contact you anyways, or not contact you at all. If you want no contact, then just reply with a picture of a kitten or something. If you want contact, then you might have to put up with the occasional inane question.