Question
One of my clients stop picking my call, did not return my call, should I go their home and knock the door? it seems very foolish, I am in the middle of the work, I want to complete the work and receive payment, the clients gave me 25% initial payment, do I have right to collect the rest of design fee?
This is the only client that husband and wife had totally opposite ideas, every meeting I spent almost four hours with them, I was patient to listen, it was very hard to work on that project, that is the job I spent too much time and not make enough money. Can I ask client real answer why they block me? Did I do something wrong? I know I will never get the answer.

Should I respect their choice, not to bother them, forget about it?

One more question, how do I prevent this happen again?  

Answer
You were wise in getting an initial fee up-front.   Working with difficult people is a fact of life, unfortunately.  If you are working on their home, knocking on their door sure seems reasonable.  After all, you're in front of it anyway!  If you are performing according to an agreement, then you are entitled to an agreed payment.

Did you do something wrong?  I don't know.  Is the agreement in writing?  The best approach is to get it all in writing.  There is something about having it all down on paper with a signature that keeps everybody (Husband, Wife & Contractor) in line.  Every time they come out and wave their arms to make a change, have them put it in writing, sign and date it.  I know it's a pain, but it's a lot less painful than what you are going through now!

Since I don't know what state you are working in, or if you are a licensed contractor, I really can't help. Most states have laws called Mechanic's Liens that are pretty effective in getting contractors AND in some states designers paid for work performed.  Check with a local building trade group, or attorney in your area.  Be careful, as unfinished work could be considered "damaging" the property - I'd hate for you to have to give back what little they have already given you.  It all depends on the state or local laws.  Consult a local professional.