At least for me, the main reason for metagaming is a feeling of control. When you feel like you can't decide what happens to your character, you might make them overly powerful or instill them with knowledge that they shouldn't have.
I, personally, have had problems with this and I feel that letting the person in question have a clearly defined 'bubble' that they get to work in helps a lot.
You also need to understand that sometimes this comes from people being enthusiastic about their characters and wanting to show them off, or simply from a place of inexperience. Sometimes they just want to show something off, or they have unrealistic plans for building the world, or their character, that the GM is not okay with.
Sometimes, they're trying to be unique and stand out. Especially with bigger roleplays, this is hard to do because everyone wants their characters or backstory to be unique and special, and often end up one-upping each other over and over again because they are trying to stand out. Inexperienced roleplayers make this mistake a lot, I feel, and end up creating a character that is really powerful because they feel a pressure to be as good as everyone else.
In conclusion, I feel gentle reminders are a good way to deal with a situation, and, by all means, have the boot ready, but if they're still learning, give them a chance to. I think with any problem you need to peel back the proverbial curtain and see the root, instead of dealing with the symptoms.