Oh, I misunderstood what you were talking about. You mean remove the sidepipe from the header. The collector is the part that is welded between the merge pipes and your muffler.(at least this is what I think you are talking about) It sounds like you have slip on sidepipes, with a spring or springs that keep them on. Usually there is one bolt that pulls the two pieces together as well, look all around the pipes for it. If the bolt is there, of course remove it along with all of the springs. If your pipes have been together for quite some time, sometimes the carbon will kind of glue the two parts together. This is when your rubber hammer comes into play. You'll just have to try to wiggle your sidepipe while tapping the pipe off the header with the hammer.
Good luck....
Tim