Well you already answered your concern --a self-moderated thread is essential if you don't want irrelevant answers and locking the thread is a good action if you have been satisfied with their response.
For me --it is a discussion board and as long as it discusses relevant to your questions does not matter if the response did not help your question directly. People are free to give their opinions as long as it is related to your question and that board called ^{[gambling discussion]} to discuss their opinion.
However, if there is an off-topic reply or response, just hit the button ^{[Report to moderator]} and the moderator will surely delete their reply.