I think that's one of the biggest differences between the professional designers and everyone else. They have systems in place to screen out these type of clients people.
They won't start working on the project until they have provided sufficient information and detail on what it is that they want and provided sufficient information with which to create a proper design brief, all the while requiring the client to sign off on decisions made to ensure they don't backtrack. As well as requiring deposits and many, many other little tricks to make working with clients easier and safer, they have the luxury of being able to turn away less desirable clients or projects. So for anyone reading who is in a similar situation, just keep building up your portfolio and the rest will come!
On the subject of clients ruining good design with their ill-informed design decisions, not all is lost and you can still get a good portfolio piece. Once the project is over and done with, and you've recovered from a couple of nights of heavy drinking, simply pick your favourite version of the design and work on that until it is completed to the best of your abilities. Now when you go to write up your case study, present this as one of the concepts you developed during the project and treat the rest of that case study/portfolio piece as you would any other project. You can even include a section at the bottom explaining that whilst you had developed this version, the client chose another direction, and then you can post up the image of what they chose - sorted.
There is an argument to say that a large part of our jobs is in educating our clients to allow the to make the best decisions, but often clients are simply not willing to learn and when the budget is low, or it's for a friend of a friend it's probably not worth your time to do so.