Since there are no definitions, I think examples are in order. I'm not going to write an essay explaining the difference, but this might help.
Here is a source on Conflict Resolution: https://www.skillsyouneed.com/ips/conflict-resolution.html
So let's categorize some stuff:
A good interpersonal solution for conflict might be:
Work with the other person and come to a common understanding of your shared objectives
The specifics of which can vary.
But, more importantly, that solution does not give any advice on particular Interpersonal Skills
Interpersonal Skills apply to many Interpersonal Solutions.
Different solutions involve different skills.
Takeaway: Many questions ask about finding the proper interpersonal solutions, and many answers simply suggest an interpersonal solution. Those are normally too broad or opinion-based. It's better that the question already have an area of focus for a solution, so that the answers can focus on the specific skills involved in whatever solution they choose to address.
Note: It's okay to include interpersonal solutions in answers, if they are supported by skills and techniques. Sometimes the solution decided in the question isn't perfect, and needs adjusting in order to maintain proper interpersonal skill. Ideally, though, the question needs some kind of direction as to what solution(s) are being considered and asked about.
Note 2: I do not mean to say the question always needs a single solution in mind. But rather, a direction for answers to focus on a specific area of solutions. A question with 3-5 different answers is generally fine. But generally they should all fall under similar directions for addressing the problem. We shouldn't be seeing two answers like "Tell him your sorry" and "Tell him he's wrong about [X]". The question should have already specified what the OP wants to convey, or at least how the OP feels, invalidating at least one of those answers.
Addressing Specific Examples:
Putting up a sign
While this is an interpersonal solution, it is off-topic on its own. There needs to be some explanation of the skills involved. What skills are being tested.
Maybe putting up a sign is polite? Okay, then if the answer explains why and how it furthers the goal, that's a good IPS answer!
Technically an interpersonal solution. But not really showcasing any skills, in particular.
If this answers the question alone, the question was probably too broad. Perhaps if the question asks how to talk to HR about [X], a proper answer could be formed around it.
Putting up a fence
Perhaps a good solution, but does not involve interpersonal skills and really isn't much of an interpersonal solution.
If this actually answers the question, the question was probably off topic or too broad for the website.
"How do I keep out the neighbor's dog?" is not an IPS question.
However, it has many IPS solutions. If the question was narrowed down to focus on one, it could make for an excellent question. But we can't just change questions. What if the OP really just wanted to put up a fence? The posters need to take this responsibility of knowing what they want to do.