Spanking seemed to work fine for my kids (something I needed, but didn’t get, by the way), but I realize other kids have different needs. In any case taking them to shrinks will leave a paper trail that can only come back to haunt them.
Many kids do improve by spanking, but ADHD is a brain structure problem. People with true ADHD have a smaller reserve of concentration and they have a poor executive ability to put less important stuff away and concentrate on what is needed.
Much of it can be addressed through proper strategies, like putting all other homework away from the homework you are doing, because you have poor executive ability to not touch the other homework until you are finished what you are working on.