In practical terms, devotees have to struggle with the instruction not to speak prajalpa. For example, we are not always in control of how particular conversations go. Sometimes we may begin by speaking Krishna-Katha but others introduce gossip into conversations. If we are in a position to instruct the devotees speaking prajalpa - that is, if they are our juniors - then we should explain to them the dangers of speaking prajalpa. If we were addressing our seniors, then the best thing to do would be to submissively ask about the purpose of the discussion. Perhaps what the senior devotee is speaking fits Bhaktivinoda Thakura's criteria for good reasons to speak prajalpa. But we should ask respectfully. If we think the senior devotee does not have a proper understanding, then it would be better not to instruct him but to inform a higher authority - someone who will be capable of dealing with the situation.
Lectures from a disciple Vol 1, Pg 440, 2nd paragraph onwards.