I believe there are two parts to this that lead to a potentially enforceable policy:
Questions must ask for problems requiring interpersonal skills for an ideal resolution
Using the example question linked above, if the person knows who the offending micturater is, then interpersonal skills are required to resolve the issue. Since the offender is unknown, however, asking about leaving notes is not about using interpersonal skills to resolve the issue. At best, the only on-topic portion of the question would be "is leaving notes for an unknown person an effective means of communicating?". At worst (as I believe is the case here), it attempts to disguise an off-topic question as being on-topic by suggesting an interpersonal solution when one isn't even practical.
In my opinion, interpersonal skills require, at a minimum, being able to communicate directly with the other party. Leaving notes for an unknown individual in the hopes that they will see the note, and read it, is an indirect method of communication, and therefore should be off-topic.
The other issue is what is acceptable in an answer.
Answers should address the use of interpersonal skills to resolve the stated problem, or offer a detailed explanation as to why the resolution being sought is a bad idea
This is harder. Due to the complexity of interpersonal relationships, we can expect to see a lot of background information provided for context in the questions. This is generally helpful, but it also provides a lot of temptation for sidebar discussion, or answers that focus on the context, and not the actual question.
For example, I recently asked how to communicate with the father of another child. Many of the comments are focused on aspects that aren't really the question (what I should say to my son, what I should say to the teacher, etc.). This isn't a problem, because comments are temporary and subject to deletion, anyway. If it doesn't provide clarification to the question (or an answer), it is noise that will likely get cleaned up. More problematic, imho, are answers which address something other than what was asked. For example, this answer, while providing advice that is pertinent to the context of my question, makes no effort to address the interpersonal issue that I'm actually asking about. It makes not a single mention of the father of the other child.
Answers like this, no matter how well-intentioned, should be deleted as Not An Answer. The same applies for any answer that doesn't provide a solution based upon interpersonal skills. "Move to another school", "enroll your child in martial arts", or "call a lawyer" are not valid answers, because they don't involve interpersonal skills. "Talk to the teacher", or "talk to Walter directly" should also be deleted, since they don't involve the specific use of interpersonal skills the question asks for.
However, something like "Don't talk to Walter's father, because x, y, and z. You should instead work through the teacher" would be fine, because it directly addresses what was asked about.