As someone who hosts these types of shows, you have 2 forms of "know it all's" - Pros and cheaters.

Pros are the people that just do trivia nights. I have some regulars that play 7 days a week at different venues, using their winnings to offset their food and drink costs. They know an ungodly amount of things themselves or they have a team of people with high levels of knowledge in different categories. You'll find a lot of retired computer and science people in this group. I have a regular group of professors from the local university. They span over almost every discipline. They come about once a month and I haven't seen them not get at least top 3 every time.

The second group, cheaters, can go fuck themselves with a cactus dildo. You'll have probably 5% of a show being cheaters. This includes people that cheat the entire time and people that cheat for one or two questions.

Sadly, not much can be done about the second group. People will always cheat and venues are extremely reluctant to kick anyone out since it could affect their income. Plus, you have sneaky cheaters. I have one group that I know is getting answers from somewhere. It's 2 people and they seemingly know everything. I suspect they have a friend at an earlier show feeding them the answers or someone in this company is leaking answers to them.

Anyways, you didn't ask but I felt like posting that so enjoy ¯\_(ツ)_/¯