Per my observations, a lot of such posts come here because of misleading comments at Stack Overflow.
For an example, here are just few recent cases at SO that directly led to users posting very low quality, outright off-topic questions because of comments directing them at Programmers: here, here, here, here.
Notice like you suggest will definitely be helpful in stopping at least most reasonable misguided newbies. However, I believe that we need stronger measures in cutting down the issue at its source.
As an example, in first case I listed, recommendation to post at Programmers was given by a user with 15K SO reputation. If a newcomer gets such a personal advice from such an authoritative user, would this "override" any automatic notice they will see on their way? you bet.
The right way to go would be to find and delete as much as possible misleading comments at Stack Overflow. "Finding" part is technically easy, all it needs is tweaking one of Doorknob's queries and studying its results. We did something like that this Spring when cleaning up "+1 great post" comments at Programmers. Searching for "programmers + belongs" will likely show us the worst offenders, although "programmers + career" and "programmers + recommendations" will likely work well too.
Deletion part is tricky and requires help from SE developers. A while ago, I "tested" it - I flagged several comments using "Other" flag with verbose messages explaining why these should be deleted. All of my flags went through successfully, however I learned along the way that this way is terribly inefficient. Involving diamond moderators and writing verbose explanatory messages for hundreds, possibly thousands garbage comments like "ask careers at Programmers"? give me a break.
- What is needed instead is to add "programmers" to the list of "sensitive" words that trigger comment deletion with a single flag. That way would let us cleanup harmful garbage with a single "too chatty" or "not constructive" flag. (Optionally, it would be good to exclude from sensitive heuristics comments that refer famous toilet bowl guidance but I don't insist on that.)
Another issue where help of SE dev team would be much appreciated is "other" close reason messages. You know, I am active close reviewer at SO. I focus strongly only on most terrible questions and totally skip the rest (except for infrequent "known good" audits, to keep more system faith in me). But even in these - I stress - totally, absolutely awful - questions I sometimes see "try your garbage at Programmers". I don't even dare to think what happens in less awful (but still totally inappropriate) questions at SO.
- The way to stop this would be to block close messages containing word "programmers" - SO close voters willing to "help" askers like that should totally drop that and try
jQuery their luck flagging for migration. Hopefully mod declines will teach them something at last.