I know it's harsh and everybody is trying its best, but sometime (because of the microphone, regional accent, unusual pronunciation) the quality of the contribution is bad or does not match our expectations.
We need to be able to block some people that prevents us to get answers from contributors we consider more valuable to us.

Throwing some ideas to ease my suggestion:
Perhaps it should always be given a reason to let the other know what to correct (if it's microphone related for instance). It could be also limited to a fixed number of people (perhaps no more than 5 ban). Or maybe proportional to the quality of our own contributions...