Here is my personal take on being a thread shepherd. Any and all activity taking place is a good thing.
Don't be so picky in terms of off topic stuff popping off in your thread, like any good conversation there should be sidebars.
The activity taking place will hopefully provide you with leads that you can use to further expand the topic you have opened the thread about, if you are listening to what is going on you can dialogue with the sidebar and this may be more fruitful than you specifically demanding folks adhere to your thread topic.
Also, whenever someone demands folks stay on topic, it kind of makes them look like a d!ck, just saying bro, so ease up and let the thread go where it will go. Peace