Someone made a suggestion this weekend to make it difficult to participate in the bad (off-topic conversations).
Off-topic conversations are good. Actually, they're the best. Off-topic conversations show that members are bonding beyond the topic matter. You should be celebrating, encouraging and participating in them.
How does a friendship develop? Typically, two people meet through a common interest. They talk about the interest. As they build trust they talk about other things. They disclose more information about themselves. They get to learn more about each other. They build a close friendship.
Communities are the sum of the relationships members have with each other. If you can only restrict them to talking about the initial subject matter, you restrict those relationships. You weaken the community.
You don't want to do that.