You can't help someone who won't let you help them. It's frustrating as hell, but there are those times when you need to force yourself to walk away and let them figure it out on their own.
My daughter in law is one who refuses to believe anyone could possibly be wiser or have more experience in ANYthing than she does. (Damn kids think they know it all. Am I right?)
The funny thing is that a week later, she will have done exactly what was suggested in the beginning - but make it seem it was her idea all along. Eh, what the hell? She got whatever the problem was solved in the end, so it's a win, I suppose.
So, in conclusion, no. I don't believe you are able to help someone who needs help, but refuses it for whatever reason. As long as the issue isn't harmful or life threatening, just let them figure it out on their own. It builds character & teaches them. If, however, physical or emotional harm is part of the equation, I'd step in regardless of their refusal of help, and do whatever is necessary to ensure the safety of all parties involved.