The underlying issue is psychological and is similar to counselling people. Basically by responding to the problem you are, in the mind of the requester, taking ownership of the problem. So if your solution does not work perfectly or is only a partial solution, the requester then gets agrieved at your failure and demands you provide a solution that does work.

If you are going to answer then you probably have to give some alternative approaches and stress that it is their problem and they have to think it through to get the most appropriate solution.