"What is the right thing to do with posts like this, given that replying to them is at best a waste of time"

We have a small number of consistent abusers, both posting asking for help while wasting our time, and handing out frankly awful/dangerous advice (in terms of careers or otherwise) while pontificating about their frankly imaginary or delusional expertise. These posts should not be ignored, rather challenged/rebutted as appropriate. To do this rationally takes time, effort and restraint.

In response to some of your other points.

1. That's up to you, however consider the future uninitiated reader. Sane responses/rebuttals work better.
2. People can post anonymously. Fortunately the initiated can usually quickly spot our distinctive trolls/abusers based purely on their content/ramblings/various other issues.
3. Since repeat offenders are unable or unwilling for whatever reason (see above), consider any educational response is more often than not in these cases going to fall on deaf ears, however, future readers will benefit from educational responses.
4. Under no circumstances should this occur.

Part of the problem is taking the time to see the bigger picture. With some of the worst repeat offenders the effort required is fairly minimal, however as noted in other responses, and their subsequent links some detective work, joining of the dots, is required.