It depends on the context you want to use them. Stopping the feedback by mic placement, or blocking (you can get studio mike sheilds which could be used in a "church hall" ) arrangement. Good mikes like the shure sm57 are reasonably resistant to feedback (unlike a lot of condensers). My acoustic guitar box has 2 feedback blockers - a notch filter and a phase shifter. Both can help in different circumstances. I couldn't define which - I just try it and see. Active noise cancellation techniques are usually for removing a fairly constant background noise rather than dealing with a feedback loop I believe, but I happen to know a leading expert on the subject, and I'll ask him if you are interested.